I'm just not seeing this. Why make this a multi-stop trip with associated wait times between each leg when you could just get on a bus at the airport and get off at your resort?
Because not all rail passengers will be coming from MCO. There are plenty of one-stop services that will take you to/from WDW hotels to MCO, but not many that will take you there from West Palm Beach, or from downtown Tampa. (Actually, make that none from downtown Tampa. Before he got a car, my DS who was attending college in St. Pete used to have to take an Uber to downtown Tampa, then an Amtrak train to downtown Kissimmee, then another Uber to WDW if he came to join us on visits. It tended to cost about $140 each way. Most of the time he paid a friend to drive him to Tampa, and then we picked him up in Kissimmee in our rental car, which shaved the cost down to about $70 r/t.)
Also, many if not most adult foreign visitors will probably prefer to take a train rather than an airport car service, because they are used to taking trains to/from airports, and it would be much cheaper for small parties.
There are (VERY expensive) car services that will take you to WDW from TPA, but no regular service from the Tampa area or Gulf beach communities, and I-4 from Tampa can be very hit or miss in terms of transit time thanks to the traffic. It might take 90 minutes, or it might take 4 hours if there is a wreck on I-4. The train should be much more reliable on time, even limited to higher-speed rather than high-speed.
Timewise from Miami, a flight to WDW is typically going to take 70 minutes gate-to-gate, plus around 90 minutes at either end, for a total of about 4 hours, at which point you still have to get to WDW from MCO. Brightline is expected to shave an hour off that, and deliver you much closer to the tourist areas, for about $100/pp. For people who for whatever reason don't wish to drive, that's a decent deal, especially if they are destination-chaining and only want to take the train one-way. Typical cost for a flight from MIA to MCO is at least $150 if you have a bag. (Most Miami locals will probably drive their own vehicles, but tourists, especially foreign tourists from the very lucrative South American markets, won't have that option.)
FWIW, I always tend to take trains in from airports when trains are available, especially when traveling alone. In cities I typically walk from the train to my hotel if it's daylight and only a couple of miles. I feel more comfortable in buses or trains than cabs, and it's usually much faster and cheaper than the road options. BWI to Washington comes to mind: there is a shuttle that takes you from the terminal to the train station, and then from there most people take the train to DC. In Chicago the airport trains take you to the Loop, and then you're on your own the rest of the way.