I cannot speak for TSMM because I don't know what is involved with gutting a building and creating a whole new space. I do, however, know that Soarin is a whole new attraction that only owes it's entry point to an existing pavilion. Hardly the same thing as what would be called adding to an existing attraction. Besides constructing within an existing building that wasn't originally designed for the new attraction, can easily take much more time then starting from scratch.
Another avenue of confusion comes from where the timeline starts when building an attraction. I feel that many here decide that the amount of time it takes to build something starts at the time they announce it. Wrong! There are millions of preliminaries that must be dealt with before construction actually begins. I will allow that from the time the first land and base clearing starts would entail time of construction. Previous to that there may be all kinds of roadblocks and problems that must be solved, some of them even during construction that we know nothing about. Permits, survey, water tables, snotty co-sponsors (yes, I'm talking to you Mr. Cameron), all kinds of things. If you decide to build a house the time of construction doesn't include how long you take to get started. It is from the time you actually physically start building. Why Disney "seems" to drag their feet getting started probably has more to do with the layers of bureaucracy that must be walked through before hand. All business are a ruled by committee now, not the good old, one person has the say and away they go. Universal has been running from a behind status for many years and are trying, somewhat successfully, to catch up. Disney knows that no matter what they do, they have limited space, they are starting from way back and even if it takes 5 more years to redo WDW, once they do, poor old Universal will be behind the eight ball again.
All I'm asking is that people use common sense and understand that construction isn't a snap of the fingers and there it is. Splash was mentioned as taking "almost two years to build" and that was a basic flume ride with stuff built around it. Nothing that required a lot of creativity. The main part of the ride is supplied by gravity. Big whoop! Does anyone know how long it was in planning and engineering before construction was even started? No, because they didn't announce anything until it was ready to go. It was mentioned that it took only three years to build EPCOT. Does anyone know how many years it took for the R&D for that project? Is anyone able to understand that the entire park was being built simultaneously? Doesn't anyone see that it means that it took 3 years to build one attraction and landscape. All the attractions took 3 years, it's just that they were all being done at the same time, and still, there were many attractions not open on the opening day. Wonders of Life, wasn't there. Horizons wasn't there, The Seas wasn't there, Imagination wasn't open, just the 3D movie. Take each one of those attractions and add up the total time it took to build them if they were done separately. I think that you would find that everything is pretty much on the same time scale as it was back then.