If given that choice I probably would have gone with what we're getting, but I completely agree that Guardians has absolutely no place in Epcot. The property should have gone to the Studios if it had to go anywhere.
WDW deserves an attraction like Tron, but I feel like the Tron attraction probably deserves slightly better than what it's getting in WDW. Hiding it behind Space Mountain seems weird, it would make more sense in the land to pave a path between Buzz and COP and build it back that way. Though they would of course HAVE to theme the building then, or risk killing the view of the park from the expensive Contemporary hotel rooms. I wonder if that's part of why they didn't do that - it sounded for a while like they were looking at building there.
Trying to position it to be "blocked" by Space Mountain doesn't strike as an elegant move, though. Let's pray they wise up and do something to make the showbuilding less of a brick. There isn't a single meaningful illusion-breaking showbuilding visible in any guest view from outside the MK, and this would be an enormous one to start with.