Not quite sure if these are veteran bands in quite the same sense, but they’ve basically been going forever:
Jimmy Eat World - Futures was one of those strange albums that was pretty great (aside from one or two absolutely dreadful missteps) and simultaneously indicated everything going forward wouldn’t measure up at all.
Alkaline Trio - Good Mourning was another one like that, though more of a mixed bag. It suddenly started feeling like a schtick.
Everclear - Songs from an American Movie has some moments but from what I’ve heard and gathered, everything after has no reason to exist.
Our Lady Peace - Spiritual Machines was a great album but then the record label took over for Gravity and that was that. “Somewhere Out There” and its even worse music video signaled the end of a secretly very good band.