TSM will likely be a particularly serious problem due to relatively low capacity coupled with extremely high demand. There are only enough slots for a fraction of all the guests in the park to ride it on a typical day, and an even smaller fraction will be FP+.
Even if Disney doesn't reserve slots for day of, many FP+ "reservers" will be locked out of TSM if they aren't early to the reservation window and will have to choose to reserve a different attraction in that category. If Disney does reserve slots for day of, then even more of those people trying to reserve earlier will be disappointed.
Yes, TSM is a problem even now, but I think most people can reluctantly resign themselves to "first come, first served". But with Disney promising their bright, shiny, and new advanced reservation system to on-site guests and others who get to reserve early, I think people will be less forgiving if they are shut out of a ride they really want.
