The thing is, they know DCA is fine. Before Marvel opened there wasn't a problem with the park for an average day guest. There are enough E-tickets to satisfy guests, there is really good food, great entertainment, festivals for part of the year, and there's a guest favorite nighttime spectacular at night. And APs don't spend all day in DCA anyway. I know that we all have a variety of opinions on DCA, but we are not the target audience. The point is that the average guest, since 2012, has been satisfied with their day at DCA prior to Avengers.
Really the only point of Avengers Campus is to add Marvel to the resort. And Avengers Campus Phase 1, while underwhelming, will do that. It will give more merchandise sales, it will add a much-needed family attraction to this park, and it will have some great entertainment. And that is enough. Disney doesn't need this E-ticket to achieve its goals of making money and featuring Marvel in the resort. Will the land benefit from an E-ticket? Yes. But do they need one to achieve their bottom line? Not at all.
As for research and development, there are countless attractions that went through much much more development and got canned (Western River Expedition, Discovery Bay, so many Epcot pavilions, Beastly Kingdom). They aren't afraid of canning an attraction because they know that for as much money they have spent on research and development, they would spend quadruple to actually build the attraction.
If the land doesn't meet their expectations or performs poorly 5 years from now, we might see this Avengers E-ticket resurrected. But they don't necessarily need to build this now and they really don't have the money. If they don't have the money to finish building TRON in Walt Disney World, which is already 3/4 done, they don't have the money to build an entirely new E-ticket attraction from the ground up.
That's just my thoughts and a very long rant
. Feel free to comment if you disagree.