Yes, it would be a bit of a change. I mean current state they have 'x' available until they are not, no reason they couldn't have reservations limited to 'x'. Say if they have 100 at MK, make 75 available for reservation, 25 for walk ups. once 75 is done, allow no more reservations. no reason to move them around.If they did that, they would need an efficient and working system to manage and save the reserved scooters, including IT systems AND locations to hold the reserved scooters. It also adds a bunch of operational issues.
They would need to manage the possibly changing number of ECVs at each location based on the reservations. For example, let's say they have 100 (nice round number) at each park. If the number being used by those with reservations is less than that per day, it is not a problem, however, if the next day, 110 reserved ECVs are needed at MK and they only have 100, they would need to move 10 to MK somehow, thus depleting the fleet somewhere else. Sure, they could setup a warehouse on property with a fleet of ECVs as well as delivery trucks, train drivers and figure out the logistics and space for park storage etc, but why do that when the demand for what you're suggesting is likely minimal and offsite companies already provide a service? or just having an inventory at each park first-come first-serve is working?
From my purely subjective view point, about 90% of the scooters i see in the park are off site rentals. IF a company can make money off of renting them for $20 a day, i am sure the price disney could demand is much higher. And the buses would be a much better experience. Our last trip, about 2/3 the folks didn't sit on their ecv during the bus ride.
While I know Disney gets their cut of the "approved" rental companies, that still isn't as much as they could make. HOpefully i won't need one for a long time, but i would much prefer to rent from disney at the park, maybe that is just me.