Is it a zero sum game? At top programs, players graduate, get drafted. Take a UM for instance - Golden & Wolf graduated, they went out and got Mara & Yaxel (and others). Yes, on occasion high majors will move off players that don't meet the bar (like a Nowell), but not sure how that sets up a VCU to make a significant tourney run or to be a cinderella, getting the scraps from the high majors. Not zero sum. It's a replenishment system in most cases.
So they get 4 stars who want PT, and then transfer out to a top program? Not sure how that helps mid majors. What mid major this year is really impressing you, and has shown S16 potential. I guess Utah St & St Louis are that? If you look at last years portal class, of the top 100, I don't think one kid portaled to a mid major. Grand Canyon may have gotten someone late.
Exactamundo. Sure, there will be an outlier here and there, like a Will Wade at McNeese. You can bet as soon as a good mid major coach makes a run, he'll also "portal" up to the next high major opening (Devries).