Seconded. TBBT works in part because it is an ensemble show - there were five characters (and then seven plus a few regular supporting characters like Stuart, Kwipke and Wheaton) which meant that even if we found some of the characters annoying the full focus wasn’t on them. *Friends *worked the same way. Extract one character and build the entire show around them with only supporting characters and the shtick gets old pretty damn fast.
Sex and the City had The Carrie Diaries. Looking at the bottom of the Wikipedia page for the latter leads me to this list. Not all sitcoms, of course.
To a degree, yes. But take away Sheldon and the show flounders. In Friends, the characters were all equal. But TBBT is essentially the Sheldon Show.