Definitely agreed that the Star Wars and Avatar sequels almost definitely won't make as much as TFA and Avatar 1. With TFA, there was an amount of hype and buildup there that we probably won't see for another film for a long time. With Avatar, a lot of the technical achievements, while impressive, won't be as novel as they were with the first film.
All of these films are going to make a ton of money, just not "holy shit that's more money than I thought there was money" money.
There's no way Disney moved Star Wars to Decenmber without already knowing before hand that Avatar had been pushed back. Disney wouldn't want the competition, regardless how great the movie may be. And Avatar is getting it's own land in a Disney park that they were trying to coincide the opening of with the sequel's release. Disney was probably made aware Avatar would be being delayed for weeks now, maybe months.
This. While it would be a fascinating box office match-up, neither series benefits from having another film of that scale in theaters at the same time.
If Episode 8 does the same business as 7 - it'll plateau by early January. Avatar 2 could come out on January 18 instead.
While certainly not impossible, I'd be very skeptical. January tends to be more of a dumping ground than a time to release a film in a prominent series.