Here's our theory. The speedway is right next to the main parking lots at TTC (ticket and transportation center, aka, the Magic Kingdom parking lot). In fact, that proximity is part of the problem with the speedway, back when they actually had races there, there was obvious conflict between people arriving and departing and parking for the duration of a race vs. going to the theme park(s). The races went away a long while ago relieving the problem. I'm unable to find any data (maybe you can do better than me?) on how many days of the year TTC is filled completely, but I bet it happens occasionally, and will happen more and more. So. Maybe extend the parking lot and the trams -- and then use that additional parking to block off some of the closer in space, then build garages. The net effect over time would be to both increase total spaces AND reduce tram travel on most days of the year (that is, you wouldn't run the tram around empty lots, and people could walk in from closely positioned garages).
Disney has started building parking garages, notably at Disney Springs. In the past, they just flat out wouldn't, and there are a lot of people who have all kinds of weird ideas about garages being ugly and evil (really, compared to acres of parking lot? That's an interesting argument! But people complained when the lots had a name switch to Heroes and Villains at the top level, and then character names for subsidiary sections, which was a pretty reality based way to ease cognitive load on visitors) and how that would somehow mess with the transition of being in your car to being at the park.
I'm inclined to believe that the logic of garages will win out in the end, despite the But Walt Didn't Want It That Way argument. Even MK at WDW is finally serving wine and beer (at Be Our Guest, IIRC), and Walt sure didn't want that. But Time Will Tell. An alternative possibility is a better location for parking buses.