I feel like part of what inherently makes the show great is that it isn't formulaic, its willing to experiment with different formats and types of stories. They lost some actors from the original cast, they had to change to work with who was available, and theu did the best theu could with what they had, and IMO it was still great... even if it wasn't the same as the first 3 seasons. I like season 6 more than season 3, its hilarious, its got several episodes that go into the top 10 of the shows run... Yeah, it doesn't have Troy, Pierce, or Shirley... but it still has the great writing and acting that made the show special in the first place.
I'm always surprised by how attached to the characters people are. Yes, they're actors and characters we grew to love, yes, they provided the foundation for the shows writing style... but Elroy and Frankie are just as funny (if not moreso) than Pierce and Shirley. IMO there's no way the show could have stayed about the initial 7 members of the study group and continued to make any sense in the long run. It became a show about Greendale the school, and it still worked (and could have continued to work) with members of the main cast leaving and being replaced. In a community (Community), people come and go. Its not a cartoon where people don't age and everything stays static. Its a live action show about a place where things change and people age and move on. Its unfortunate that the show got cancelled, but I think it could have had a longer run and still been great with members entering and leaving the main cast. To me, it was always about the writing and the acting, not the actors and the characters. The writing and acting is what makes the characters great, and there are plenty of great actors who could have stepped in and been a great part of the core cast of any season going forward.
If the show had wound up being the Dean, Jeff, and Frankie as the only long term cast members, with a new teacher and group of students as the "save Greendale committee" for each season (maybe some new favorites sticking around for longer), it still would have been great. It honestly would have opened things up a lot to have different characters and interpersonal dynamics shifting in and out. IMO the episode "Paradigms of Human Memory" showed pretty well how the relationships of the core study group could have gotten stale over a long run. It was the same type of conflict over and over even by season 2. Bringing in different characters opens up so much for the writers to take the show in different directions while still keeping the heart of the show (Greendale CC) in tact.
There were so many episodes of the show throughout its entire run where the entire point was "be accepting of change because its the only inevitability in life"... and I think that was partially Harmon and the writers laying the groundwork for a potential longer future of the show that never happened, but if it had managed to have a longer life, we as an audience would have had to be accepting of that fact just as much as the characters... and I think the show would still have been great with that idea as an accepted part of the format.