I am fully in the boat that the Mickey ride should be a new build and they should keep GMR.
That said, it seems the premise for putting in the Mickey ride is to relieve some of the stress of the crowds when Star Wars opens... thus the Mickey ride needs to open by the late 2019 timeframe that SW is expected. Putting it in the old Animation Bldg/Launch Bay area, while being logical, doesn't fit logistically since they aren't going to close Launch Bay until the new Star Wars land opens. So, you couldn't start construction for this theoretically Mickey ride in that area until 2019 at the earliest.
Of course the basic logic falls flat when GMR could help with capacity and the crowds just as much as the Mickey ride. I think it would be fine to keep GMR as is and put the Mickey ride in the Animation area -- if it opens in 2021-22, that's fine.
In the meantime, I'd really like to see them start on something else for DHS. The most logical place would be on the Indy stunt show footprint (ideally going into the current backstage and parking area for more space) and starting to build there when Toy Story Land opens.