The big money makers in Hollywood know that fan service is a hell of a currency. Give the audience just enough of what they want and they'll lap it up, particularly when it's something they've been waiting for for a long time. Whether that's something seeded early in a franchise headed towards an obvious end-point or a big cinematic meeting of talents or characters or something in a source material, fans often wait a long time for the pay-off.
Unfortunately, while you'd think that being denied something for ages would make audiences grateful to overlook when those pay-offs might not be all that great, sometimes, it's impossible not to be disappointed. After all that investment, the money shot can just end up seeming a little... unsatisfying.
Essentially, sometimes, BAD things come to those who wait...
10. How The Snap Was Undone - Avengers: Endgame
Even well before Infinity War's end sent half of existence off as dusty piles into the Great Yonder, we knew that Thanos was going to get ahold of all of the Infinity Stones and snap his fingers. The decision to break the Infinity Saga's culmination into two parts a year apart basically gave that much away - if it wasn't ending on an almighty cliffhanger, then there was no sense in the split. When that became clear, the question of how it would be done became immediately more pertinent.
The theories were many and varied. Fans came up with complex plots based on hints embedded deep in previous MCU movies. Others made up spurious leaks, wild rumours and near-criminal speculation, casting a net as wide as possible to cover all bases. When Infinity War came and the snap happened and then Ant-Man & The Wasp introduced the idea of the Quantum Realm and Captain Marvel popped up, even MORE possibilities reared their heads. It was breathless.
A rat ended up being the key. A f*cking rat. A F*CKING RAT that accidentally freed Scott Lang from the Quantum Realm and set the plot in motion. Sure, the rest of it was great, but there can be nobody who accepts the rat solution as a fitting one.