Well then it should be universal then right? which is not the case. You can take any number of tv series in U.S which are far good even now, say like True Detective or Breaking Bad and many others
yes, but even there, mediocrity has flooded in. they may have their good shows (even we have some good contemporary shows, though far less), but even there, the number of bad shows seems to have increased terribly. i don't watch the English TV shows (have only had glimpses of some), but i suppose most of the bad ones there are invariably from that one voluminous box, which is also one more gift from Amerikah to the world - 'Reality TV' (supremely stupid shows like jershey shore, catching up with the kardashians, etc., etc.). this genre can alone account for the degrade in TV-entertainment there, while the rest, likes the ones you have mentioned, form the section of shows there that are still good.
same (or perhaps worse) is the case in south america (particularly, brasil; they've been taking inspiration from their northern neighbours as well as from the daily soaps from here too; double whammy!).
compared to the shows in the 80s and 90s, which felt to be providing a 'novelty' factor to the audiences, these ones look outright bland & chaotic (i think there was one documentary too, on what was entertainment like in the retro-times/80s)!
when quantity increases, the competition gets fiercer, and more often than not, the USP of 'uniqueness' commits Harakiri by jumping out of the window as a consequence. here, we try to ape the west in many ways, and create a mess when adapting the TV show themes from the west (mostly the reality tv).
i would've personally liked for the TV to have just a few shows, and provide unchaotic doses of entertainment, instead of bombarding the viewers with batteries of them. but can't help. 'consume & consume on' is the order of the day, the resultant being what we are seeing (and lamenting!).
in the info & entertainment category, more than TV shows though, i follow documentaries and music, and in these, there's undoubtedly been a steady decline world-over!