That was because of X2's success. Sequels always gross more than their predecessors. X2 did $100m more than the original movie, but The Last Stand did only $50m more than X2. If it had been a good movie and spawned a good word of mouth, it would have broken $500m, at least.
No, not every sequel makes most than its predecessor.
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Money made =/= film quality. Unless you want to start singing the praises of Spider-Man 3 and the Transformers films...
Do you remember your own post, which I was replying to? You said it wasn't only fans that hated X-Men TLS; for instance, you said, critics also hated the movie. My point? Critics and fanboys might have hated it, but it did good numbers so it's not like everybody did.
I'm not even touching the quality topic here.
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Rising franchises, not franchises that have already reached their peak (Batman, Spider-Man) or the ones that died in the start (FF).
I'm talking in financial terms only, not quality.
Batman 89 did absolutely great, B Returns not so much, B Forever did good again and was successful, B&R was not. It's certainly more a movie-to-movie thing more than this 'if the previous movie made good money, the next will make better/if the previous movie did bad, the next will make worse' thing. If the previous movie was good it certainly helps, but it doesn't settle the deal.
People go to the movies regardless of what the franchise's peak is. Nobody knows what the peak is until the movies have already collected their money.