No disrespect, but I think it was the same, just the standard was different.
Mike and Gloria WERE "young and good looking".
I don't know...maybe...?
but Cheers: Sam was a player; Diane was supposed to be attractive.
Sure, but right alongside of them, you had Coach, Cliff, Norm and Carla.
Taxi: Jeff Conaway and Marilou Henner were supposed to be attractive; even Judd Hirsch was made out to be above the station he was in.
Not sure what your comment about Judd Hirsch means, but right alongside those folks (as well as Tony Danza), you had Danny DeVito, Christopher Lloyd and the comedian guy who died whose name I can't recall at the moment.
Barney Miller had Hal Linden, Jack Soo, Steve Landesberg and Abe Vigoda. These are people who, if put in a time machine from 1975 to 2021, are NOT getting lead roles in a network sitcom.
The closest thing to those folks that you get over the last couple decades is Richard Belzer on L&O:SUV, and he he stands out like a sore thumb (or at least he did at the start of the show before Mariska Hargitay started showing her age). Or maybe Mayim Bialik on Big Bang Theory. Heck, BBT is the poster child for this sort of thing. Instead of getting people who actually look like nerds, they hire actors from People's 50 hottest men alive and made them up and dressed them to look like nerds. It's sort of like the "pretty, ugly girl" from every late '90s/early '00s teen movie.