The 787 seems to have some ongoing issues and it's reasonable to fear slow response from the FAA if anything does happen

The 321 is sold out for years and years. Airbus is not dumb. If they move heaven and earth to get United some extra lift quick, they aren't going to miss the chance to lock in the 350 as a the 777 replacement and sell a bunch of extra 330's too.

But 330/350 is definitely two type ratings versus 757/767 being one. I'd be really curious to see how the fleet gets distributed - 330 on the east coast, 787 on the west coast?