You make an excellent point with your question, Dave. My answer/comments:
At this point it would be next to impossible, if not completely impossible, to move that many guests with rail as the only mode of transportation, especially the way the property is laid out. If it were even possible, it would take a massive system and budget. In my opinion, the best approach would have been to keep up with expanding and enhancing the monorail and possibly building other types of rail systems as the property was rapidly developed over the past 2.5+ decades or so, but obviously we can't go back in time.
However, I would have loved if the MM+ budget was instead used to enhance the resort transportation system to rely less on buses, or at least turn the buses into something that is more of a special experience and unique to Disney. Unfortunately, it's difficult to quantify the ROI from a project of that nature though so I totally understand why MM+ would win out there (like it was ever even debated..). I still believe though that long-term, there is definitely intrinsic value to updating the resort transportation infrastructure in a major way, even if those updates do not include expanding the monorail. There is probably a very fine line between just throwing away tons of money and actually improving the guest experience enough to see ROI, with a lot of risk involved and not a lot of hard data to support my theoretical claims
Back to your question and to the layout of the property...The property would have had to have been developed and laid out in a more logical fashion for rail to work as the primary mode of moving the large number of daily guests you mention. I'm sure obstacles such as suitable land or land that was set aside for conservation came into play when deciding what parcels of the property to develop with parks and hotels, but I've often been left scratching my head when looking at the way a lot of the resort has been developed after the completion of the original MK area, or Phase I.
Sorry for the super long answer/response! In summary, it would have to be massive, so massive that at this point I can't imagine there is any discussion internally about how to potentially go forward and make monorails the primary mode of transportation these days. I'm sure it may come up in discussions as part of a bigger multi-modal initiative, but not as the primary.
I'm sure you actually have reliable inside information on the subject and since you visit the parks frequently I'm curious to hear your thoughts on what can/should be done with the current resort transportation system.