Which they also did when it was high-speed rail sponsored by the federal government. That project fell through in part because of universal's temper tantrum although that was not the nail in the coffin for the project.
However It does raise the question as to whether or not they really want do this? I suspect they don't. All they really want is to make sure there isn't a train that goes from the airport to Disney. It's okay if it goes to them first and then Disney but definitely not direct to Disney. Or no train at all is just fine and hence they put up A stance that seems like they support it when really they're putting up more obstacles than there are solutions. By design.
There will be people who say they will take the train. I bet if I got a survey or asked at MCO if I would take a train to Disney/Universal, of course I would answer yes, but that is not the full answer.
The big caveat is the time, convenience and price.
I recently got back from a WDW trip and MCO to our resort (Coronado Springs) was about $65 for a lyft. I was a party of 4, so about $30 roundtrip per person, not bad. It would need to compete in that range, but probably be cheaper since I was dropped off at my resort and even better very close to my room.
Convenience, any transfers from the train to buses needs to be fast and easy and not add to the cost. (could they do some luggage transfer option?)
When I traveled to London UK (pre-pandemic) and took the Heathrow Express it achieved most of these things, but I was without my family for that so my math would change for any trip to WDW.
If I have to stop at 9 stops before getting off the train to only transfer to bus with another 5-6 stops. No thank you (for many reasons but time is the greatest). They would need to consider express trains to Disney and Universal to make this work. It is why the CTA in Chicago was not a primary method to get to the loop from ORD, way too many stops.
This study needs the end to end journey in mind and Disney/Universal needs to involved to ensure it makes sense. I suspect any study done in the way described will not actually get to this level of detail, but maybe they will surprise me.