I think there's a lot of truth in this. We also have to remember the time period when this all started getting worked on. HP was just getting off the ground, and Disney probably got nervous. DAK had been a problem for a while, and it was begging for fix. Kill two birds with one stone. With all that Marvel IP locked out (of course they'd never build anything like that in DAK) Disney took Avatar in the hope they could combat HP like that. Also with that foreign appeal they could even get Oriental Land to cover half the design costs.
An HP killer, and DAK fixer all half off? They went with that.
Then Cameron was himself, Oriental Land got ticked, Avatar is'nHP didn't end up making much of a difference to Disney after all, and Disney got access to Star Wars and Frozen.
Killed the urgency a little bit...