Brightline to Disney won’t be online until 2023 at earliest (looking iffy). Plan is to advertise train to bus from the Springs hub. Train expected to run between airport and Springs every 30 min to 1 hr.
This is going to be an exceptionally poor replacement for ME. That will not be a pleasant experience.
Unless the check-in/luggage transfer still exists and all you have to do is physically board the train and bus and not worry about your luggage. I do not see that as a viable option from Disney's perspective, though, unless they charge a pretty penny (which you hinted at).