I still just don't see Brightline's traditional model being very conducive to convenient transfers between WDW and MCO, nor do I see Disney ending DME without a free replacement, because that would defeat the purpose of the program.
However, I wouldn't be totally surprised if Disney contracts with Virgin/Brightline to run more frequent, lower-class, direct, Disney-exclusive trains between MCO and WDW as a replacement for DME. It's basically a question of whether that would be cheaper for them than running the current DME system.