I know I'm irresponsibly over-analyzing an unconfirmed rumor, but here it goes:
A couple of things -- firstly I agree with much of your sentiment.
Tron would be a very different ride -
if it even happens in Florida - a multi-launch coaster, right off, would separate it from your very accurate "70s wild mouse" description of Space Mountain.
Yes, they would both be indoors, but does that matter if they are completely different rides? Pirates of the Caribbean and The Haunted Mansion are both indoors too. Granted that's a much broader comparison, but I think you can see the point that exists within it.
Your notion that perhaps the Space refurbishment was cut so far back in order to make a Tron coaster fit in later down the road, while interesting, is really just a conspiracy theory with no basis. The truth is there was no Tron coaster talk when the Space refurb was in the planning - and worse, budgeting stages. The possibility of the
if of Magic Kingdom getting Tron is a somewhat recent development.
I'm surprised it's not been brought up yet, but for a while there was consideration going toward a full refurbishment of Tomorrowland for Tokyo Disneyland. It would have involved a new E-ticket indoor coaster tentatively named "Cybermid." In its bare bones, Cybermid was a Rock 'n' Roller Coaster clone with a high-tech theme overlay (and obviously no Aerosmith.) It would have co-existed with Space Mountain in that case, so the notion is not so off the wall, nor in that sense is it even new. This ride is merely an example and *not* rebranded as Tron. I have confirmed for certain that Tron is *not* a Rock 'n' Roller Coaster clone for Shanghai.