I think TV shows typically deteriorate because they get approved one season at a time, and depending on how the show is going, things can go downhill really, really quickly. Spiderman 3 from the Sam Raimi series is one such example. "Game of Thrones" Season 8 is another. Usually it's some conflict with the editor or producer and the writer and possibly the cast.
I don't think "The Crown" Season 2 suffers from this problem. Either that or I'm a really easy person to please, and I am, or it's both. There's some of the same level of conflict and drama compared to the first season, but now it's mostly revolving around things like infidelity and self-inflicted drama (e.g. the Suez Crisis).
There's one good episode around the Duke of Windsor and how much of a traitorous piece of shit he was (he handed to the Nazis the Allied defence plan of the Continent which is partially why France fell), but otherwise it was pretty dull. I think you can tell it's an Americanized version of a British thing. The British are masters of understatement (of which I would like to become a pupil), and Americans like drama, so it's dramatically revealing some pretty dull stuff. I like dull TV shows though, to be fair. Means I can watch without really committing.