LOL.... Not the first time I've been accused of overthinking things.
And true, I've seen Disney do some amazing overnight transformations before, but I don't think it would be cost effective for them to approve the idea of designing sets to be swapped in and out quickly in the GMR. I also wonder how easy it would be to have pre-built sets to move in and out of the attraction in a short time like that. [Think about the month+ it takes to install the holiday overlays in
Disneyland] (Oh! and For the Record... i remember seeing the Castle when it was being painted Pink before they put the inflatable candle decorations on it. It wasn't an overnight transformation either)
And Adding Fastpass+ to an attraction that didn't need it isn't really "spending needlessly". In order for the MASSIVE investment in the fastpass+ system to work, there have to be fastpasses for guests to reserve in advance. There also has to be enough of them to make people see a "value" in the system and want to use it. Currently, there is not enough fastpass capacity in several of the parks to allow the system to really work on the scale Disney wants it to. MGM has only 4 attractions currently with fastpass [that I can think of]... ToT, Toy story, RnRC, and Star Tours. Those 4 attractions cannot support by themselves the Reserve your Fastpass system for every guest who would want to visit that park, so they need to find a way to increase the total fastpass capacity at the park. Since they can't build another Tower of Terror, the cheapest and easiest way to do it would be to add fastpass to other attractions and shows.
There is also the social engineering aspect of the new system. Disney has stated that one of the ideas of the whole nextgen project (with things like it's cell phone alerts aspect) is to help distribute the crowds evenly around the park to try and make more efficient use of the overall park capacity. Lets say they have 10,000 guests in the Fantasyland section checking out the attractions in New Fantasyland, and classics like Small world, Peter Pan, and Pooh. Currently this crowd could end up being a moving clog thru the park as the day continues since Disney has no way to help direct a crowd this large. The Idea is that with Nextgen, thru the alerts.... and also the fastpass "packages", That disney could take these 10,000 people, and try and help break the group up by sending some towards tomorrowland via their next Fastpass, Others towards the Haunted Mansion or Frontierland.... and maybe even a 3rd group back to Adventureland and the Jungle Cruise/Pirates, which for many guests who still follow the clockwise/counter-clockwise touring pattern, is not part of their natural tendency to basically head back towards the front of the park from the back.