Is the current SM a maintence problem? Otherwise, I couldn't see WDW removing such a popular attraction. Putting new track into an already built structure isn't all that easy, is it? Yes, it might fit just fine but you'd have to pour all new footers for the track to bolt to the floor of the building (no?) and you'd also have to figure out how to get the track inside. Tearing down part of the wall to do that wouldn't be all that inexpensive. Plus, you're also forgetting about the current mindset of Disney management. I'd sooner see them building a whole new rollercoaster elsehwhere (maybe even next door?) than to see them replace all of SM. With this many younger and non-thrillseeking customers, you have to have some rides that appeal to a wider audience. Why would they alienate so many people and teardown a rollercoaster and put one in with loops? Also, aren't DLP's SM a little too similar to Rock'n Rollercoaster? Seems like it'd be a little too similar to build one here in WDW.
Just my thoughts,
Valentin