I am a bit biased since U2's Unforgettable Fire tour was the my first concert ever. I didn't think that album or Joshua Tree were signs of the end. If U2 stayed on course with that style of music, they might have done 3-5 more solid albums. The problem is after Joshua Tree and especially Actung Baby, Bono decided they were the Beatles and thought they could be the trend setters in style of music of the times and Edge decided that he could just get by with music trickery rather than actual guitar playing.
I think with Metallica, they decided to change their sound and boxed themselves in a corner. They just didn't seem to know where to go after the Black Album because they wanted to cater to the masses who didn't like their early stuff (at least a lot of them). And it exposed how crappy of a drummer Lars Urlich was. He is a great metal drummer when he mostly has to play fast, but can't play to an actual melody. His drumming on Nothing Else Matters may be the worst drumming I have ever heard for a major rock hit.