Here's a question that no one here may know the answer to but here goes anyways. Now that MMRR has been all but confirmed for Mickey's Toon Town at DL and it seems like such an appropriate fit for that land. Is it possible that MMRR was in fact designed for DL but was borrowed to replace GMR when They decided not to put the money into refurbishing it? Track with me, maybe they said we need something to replace GMR since we aren't going to fix it and there is going to be loads of people in 2019 at DHS b/c of SWGE. So they say "well we have this concept we've been working on that we can make work."
I'm sure that I am just connecting dots that don't actually connect but thematically it seems that this ride doesn't fit as well in DHS as it does in MTT. I was excited about having a one of a kind ride as a headliner for DHS but since it seems that they will clone it to DL, and possibly DLP, it just made me think that maybe it was always in the works for DL but got expressed for DHS instead to help with capacity and to replace GMR instead of a refurb.
Just my thoughts.
A more likely scenario intimated by our insiders is that GMR was become so stale and decrepit (which is WDW's fault for not keeping it fresh) and had issues with maintenance and the fire scene, that they had a choice: spend a huge sum to fix it, or, spend that sum on a new ride. And seeing how brand new rides bring in big crowds; And seeing they were planning on a big MK 50th Anniversary and wanting to do a Mickey ride, they decided to replace GMR and make Mickey the main attraction of the park.
Then when SWL was greenlit, they rushed MMRR into production to be open before SWL rather than have that spaced closed during the onslaught.
Now that they see how good MMRR is going to be in their prototypes, they decided to clone it to other parks and split the development cost between them. After all, "it all started with a mouse," and it would make sense for all the Disney parks to have a Mickey Mouse headliner ride.