I guess the decay in quality occurs when the inspiration for the series, and the ideas the writers have for the story, begin to dry up, the later films noticeably suffer. The magic is gone, in a sense.
However, while this does happen often, there are quite a lot of great trilogies out there. The Star Trek movie trilogy of 2, 3, and 4 are great, as is the Jak series and Ratchet and Clank series for video games, Toy Story, The Evil Dead, and countless others I can't even begin to list.
There's a lot of great stuff out there, you just have to look in the right places