They could have built a different anchor than Cosmic Rewind, and one that was better both thematically and in overall ride quality -- in some ways, Cosmic Rewind is a swing and a miss, although not because it's a bad attraction (it is, of course, not bad). People enjoy it because it's a good coaster, but it's a poor usage of the Guardians IP. More importantly, the theming in general is more or less superfluous to the ride experience. Most guests would enjoy it as much as they do now if it was just general space imagery with music and everything else was excised.
Cosmic Rewind at DHS, especially with an additional Doctor Strange attraction and a Marvel NYC area, would be a much, much bigger boost to that park than Cosmic Rewind has been to EPCOT. Current DHS is still a weaker park with less to do than EPCOT was before CR existed. It's not that EPCOT didn't need another headliner attraction, because it did -- they just could have handled everything so much better.