Only if Disney subsidizes the cost and offers a seamless luggage experience. Current situation. Grab your bags. Summon Uber or Lyft. Wait 5-10 minutes. Walk outside, short walk depending on your airline, get in your car. 30 minutes or so later arrive at your resort.
Brightline. Get your luggage. Schlep up to level 3., go to the tram for the new pod. Schelep over to the Brightline station. Pay $$ for a ticket. Wait for the next train, maybe 30 minutes. Train to DS. Then what? Ride Share? Will Disney have buses waiting at DS? Luggage under the bus? How much?
Direct service from MCO to your resort is more efficient. Probably less expensive. Bus service, even with stops at a few resorts,is more efficient.
I don't think Disney is looking to replace DME. Guests are, and will, be told what's available. My opinion might change if Disney used Brightline as the opportunity to charge for transfers. Might work if luggage was handled like it was with DME and RAC. Guests paid a fee similar to what Mears Connect costs. Disney would get a wholesale rate and might have to subsidize the service and/or charge enough for Brightline but absorb some or all of the cost of the bus service from DS to the resorts. I don't know regulatory issues. I know Disney contracts out with Mears for buses that don't operate exclusively on Disney property,
DCL DME etc. Buses within Disney property are generally operated by Disney.
Brightline makes sense for business travelers looking to go directly to the convention center.
In the future Brightline may be a big help to guests who want to fly into Tampa and even Miami. A help to guests who want to cruise out of Miami.
Brightline crunched their numbers based on Amtrak operating between Boston, NYC and DC. All 3 cities have excellent mass transit.