Never said all slots wouldn't fill. It will just cause guests to have other options. Opening opportunities for others.
As I’ve already stated - The park reservation system already prevents anyone who does not have DHS as their Park Pass for the day from being able to access RotR boarding groups. You could have 100 other attractions with boarding groups on the other 3 parks, it wouldn’t affect demand for RotR boarding groups one iota. You can’t park hop to DHS and try to get a boarding group. Nothing they are building will come close to the demand for RotR.