I know that many critics at the time attributed Frasier's decline in S8 to the coming together of N and D. I always felt that this was a simplistic view, expressed by those who didn't know about other factors, such as the departure of Keenan and Lloyd. But the more I've thought about it, and the more I've analysed the show, the more I think that bringing N and D together was the key reason for the show's decline. I think that blaming the S8 writers for 'not handling N and D's relationship well' is not fair at all. Here is my analysis of why I think so, for what it's worth:
Very few shows manage to survive a significant change in their dynamic without some decline, and in the case of N and D, the change had a fundamental effect on the show in a number of different ways. Here are the ways that I've come up with:
1. Obviously, you lose forever the comedy of N's unrequited love/lust, and D's obliviousness, which provided so many great jokes and moments, and indeed whole eps, up to that point.
2, You lose the tension, drama and poignancy of that setup too.
3. The way the 2 came together – ditching their other partners – meant the partnership had to succeed or it would seem as though they had jilted their lovers for no good reason. (This was especially true of Donny, who was portrayed as a lovely guy, deeply in love with D, throughout. And before they made her awful in Whine Club, I actually thought Mel was a far more suitable partner for N than D.) This put an awful lot of pressure on the relationship working out, leaving no room to back track if it didn't. Thus even the way they were brought together seemed designed for maximum short-term impact and drama, giving little thought for the dramatic, rather than comedic, elements that would have to be sorted out in the following season, dealing with all the hurt feelings of the rejected lovers.
So the writers are faced with a problem. If the couple's relationship goes well, it is boring, has no tension, and provides no stories. But if the relationship is too problematic, then the audience will wonder why they got together in the first place. The manner of their coming together left no room for a gradual development of the relationship to something more serious. They were committed at the outset.
4. Jane and David have zero chemistry as a couple. David looks very uncomfortable with her physically. This really does not help the believability of the relationship, especially when N has been portrayed up to that point as brimming with sexual desire for her.
5. Given how their characters were portrayed in the previous seasons, N and D are not a believable or compatible couple. Indeed, a lot of the humour in previous seasons derived from N's feelings for D came precisely from the fact that his feelings were inappropriate, that they were unlikely to be reciprocated, and that the two were not compatible. D likes her men rather manly, from the evidence of earlier seasons, and a little more down to earth. So just for the sake of believability, the writers are forced to change their characters. (This was already happening in the second half of S7, as they were being groomed for their coupling). D becomes less kooky, N less highly strung, less mannered. Essentially, the elements that made them such wonderful characters are removed. That's 2 main characters, out of a show with 5 main characters, irrevocably changed for the worse. 2/5ths of the show now not working. This makes it virtually impossible for the show to continue to function successfully.
6. Loads of comedic and dramatic potential is lost in terms of story ideas too, as we cannot have any storylines about N and D's dating lives, areas which brought great humour and stories throughout the previous seasons. Much-loved eps, including Keenan's great farces, from The Matchmaker to The 2 Mrs Cranes to The Ski Lodge, rely upon D and N being single. N dating, in particular, is full of comedic potential (utilised very well in Shutout in Seattle, for example, or Four for the Seesaw.) All these story possibilities are now closed off.
7. There are problems with the dynamic of the ensemble too. There were two main comedic roles for D in the show. The first was as the unknowing recipient of N's adulation. The 2nd was her ability to be an outsider, to enter a scene with her washing or whatever, have a strange opinion or anecdote, and then leave again, leaving the others with puzzled or baffled expressions. But once she's with N, she's not an outsider. She stays on the sofa as N's partner, and her role changes. So the 2 main comedic roles D has on the show – gone!
Comedy shows have a delicate equilibrium of elements, and sometimes even small changes can destroy the sensitive balance of comedic spark and energy, the chemistry of the overall interplay. The bringing together of N and D caused not just one but a whole host of problems. But this is not the end of it, because these problems have a knock-on effect on the show as a whole, because then the other comedic aspects of the show suddenly have much greater importance, they have to carry the show even more, and the show inevitably becomes more forced, more unbalanced. Add to this that the show was declining in comedic potential at this stage anyway, and you really have a tipping point. (S6 already showed the show had passed its peak, which IMO was seasons 4 and 5. In S7, plots were already being recycled. And what else was there? For example, the Niles and Frasier rivalry had been covered, really, and there wasn't really anything more to say about Roz's character after 7 years, there was no more Maris to make fun of etc.) Regardless of the writers, bringing N and D together meant that there was always going to be a large, noticeable dip in quality at season 8, that would never be able to be fully restored. (I don't think it ever was restored. I am nowhere near as fond of S11 as some are. The episodes lack subtlety and emotional truth (I don't believe in the Martin and Ronee relationship, for one thing) and N and D's relationship, in particular, if anything feels even more forced in that season than in earlier seasons, and D's character is actually at her least charming and most annoying in S11.)
But it didn't have to be this way. Keeping D and N apart would also have had its difficulties, but nothing compared to bringing them together. Some people say 'they couldn't have carried on the same way any longer'. Why? I disagree with this. Look at some of the last jokes we get out of the old setup, for example D's risque wedding dress, and N's reaction to it, in A Tsar is Born, shortly before Back Talk, where it all changes. Even though we're seven years in, I find these moments wonderfully funny, and so does the audience. I think N and D should only have been brought together if and when it was known for sure that it was going to be the last season of the show.
Another option would have been to 'close the door' on the whole thing. I think one could have made a really lovely episode out of N confessing his feelings to D and her gently explaining that she liked him as a friend, but no more. It would have been heartbreaking for N, but then we could have seen him move on from this, and maybe start dating other people, maybe find someone more suited to him. At least this way the show could have remained true to the characters, and only the first two aspects on my list above would be an issue. The other problems listed would not exist. It would have been a very 'real', mature choice for the show. The more I watch seasons 8-11, the more I realise I just don't believe in N and D as a couple. (I always found the previous setup – i.e. N and D being friends, but N being very attracted to D – completely convincing.) I can forget this and still enjoy these later seasons, but the truth of that is still there, and in a show where I always felt the characters and relationships to be very emotionally real, however unlikely the plots, this is definitely the biggest single issue I have with the show from S8 onwards. It's the main reason, for me, that the 'magic' leaves the show. And none of it is to do with the way the writers of the later seasons 'handled the relationship', thus I think it's unfair to say this. I think people underestimate just how much more difficult the show became to write from S8 onwards. (I've read some 'N and D as a couple' fan fic, and whilst it is enjoyable, none of it is anywhere near as funny as anything in Frasier's first seven seasons!)
Phew, that turned into a long post! I welcome any comments/thoughts on any of the above!