All just my opinion but:
Personally I can't see the monorail being replaced by buses at Magic Kingdom any time soon.
Despite the huge cost of the monorail, and people's general dislike of buses, I think the true difficulty would come in the slow loading capacity of the buses.
Buses simply take too long to load and unload, particularly when you consider multiple scooters and
ECV's needing to be secured.
If you consider an average attendance at MK to be 40,000 - 50,000 guests you would be hard pushed to move that many people to and from the TTC with buses and ferries alone. Particularly when so many people leave at key times. I believe maximum capacity of MK is around 80,000 thereabouts.
Even with the monorail's fairly fast loading time, there are still massive queues of people trying to leave the park en mass much of the time. However many buses you have in the fleet, there comes a point at which they cannot be loaded quick enough to move the crowds needed.
We already have three bus loops at MK just for the hotels. To build more bus stops would become impractical in my opinion. It's already a fair walk to the furthest loop as it is.
I think there is always going to be some level of need for a mass transit system to move lots of guests to and from the TTC and MK, particularly with the speedway closing and the reclaiming of more parking space. With 10,000 cars in the lot even with say 3 people per vehicle that's 30,000 people to move.
I think rather than extending platforms, we would likely see trains which have connected carriages which allow people to stand all the way through. Similar to some subway trains where there is no wasted space between cars. Also, with level doors (think elevator doors) loading and unloading of scooters would be far faster and less labour intensive than the current system with ramps etc.
I can't say I believe the Epcot loop is as essential as the Express loop to the TTC however. It's a nice thing to have, but there are options that could replace it. I think they are stuck with the Express monorail at MK whether they like it or not.
Short of filling in seven seas lagoon I can't see how they can get rid of the express monorail or something very similar to move the vast number of people quickly enough to clear or fill the park.