I feel like there's a degree of truth to this. Not necessarily true that they don't want to close Cosmic Rays because it makes too much money, but rather because there aren't enough large QSR locations in the park to offset its capacity. Rays serves so many guests its not even funny, often filling its dining room and overflow seating for hours on end.
Since the Stitch/AE footprint is so small, it would make sense if designers want to eat into the Cosmic Rays footprint for attraction space, and if building codes require them to update the entire structure anyway then they might as well go all in. The big issue though, as mentioned, is where do you send the rest of those hungry guests? Pecos, Harbor House, and Pinocchio are all on the opposite side of the park. Opening Tomorrowland Terrace fulltime sounds like an option, but it would still need more to supplement it because it needs to close at dinner time for firework parties.
Suffice to say, if the plan is for a Stitch replacement to overlay the existing theatre, it could come in the near-mid term; if the plan is for a Stitch replacement to alter the space significantly, it'll come much further down the line.