If the rumor on the whole thing is true, part of me feels it's a combination of things which led to it.
This is just me spit balling things of course...
So the plan was a new ride and land of Avatar, but as
@Clive mentioned it felt more like an idea when they announced it, nothing really set in stone yet and we've seen time and time again how things can change in development. That could be a potential issue if Disney was hoping to open something in a particular timeframe.
Secondly, merch. Avatar is not an IP that really moves merch. It is what it is, and depending on the situation, I don't think it's much of an issue...but it does depend on the third factor...R&D.
If a land/ride etc. is bought by multiple parks and they pitch in for R&D, I think it's reasonable to assume it's given a better chance to be built as they're taking out a good hurdle for it, and they don't have to lean on merch to help it out. But here's the thing with Avatar, I do recall in the past it was shopped around to the other parks...and no one else bought it. If DL has to foot the bill for R&D...I can see how some folks might be hesitant to do so for something that doesn't really move merch.
Now compare this to Zootopia which already had its R&D taken care of thanks to Shanghai. You want it? Put the money down, make the space for it and it's yours in a reasonable amount of time.
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As for Zootopia potentially coming to DCA, I'll admit it's one of the stateside parks where I'd be fine with it. I see this like Spider-Man and Transformers in Universal Orlando, while obviously Transformers was made for parks that don't have Spider-Man...ultimately they're located in separate parks.
And as for DLR, RotR and MMRR are in Disneyland...and Zootopia would be in DCA which currently doesn't have a trackless darkride.
Another reason why I'd be surprised about Zootopia replacing Avatar (especially if it includes a straight port of Shanghai's ride) is that, based on the reporting around Villains Land, Lightning Lane seems to have changed the financial calculus justifying why and how rides are greenlit in the American parks
Regarding the changes to WDW's Villain's Land, from the sounds of it the changes were imposed by ops at Magic Kingdom who didn't want the big E-Ticket to be an outdoor coaster which'll most likely shut down during inclement weather which happens often in Florida. In ways, that's a Florida-specific issue and I don't think that would be the case if that was being built here in Cali.