Anyone who is a sophomore and older right now will be gone by 2021. That doesn’t leave too much talent left on the Illinois roster. Of course, more players will come in over the next few years, but it is hard to judge who this program will have available.
Out of the freshman on the roster right now, Ayo Dosunmu will be gone, probably, after next season. I don’t think Giorgi Bezhanishvili is lasting four years with the way he is playing. I assume the NBA will be calling him before his senior season. That means two of the six freshmen will be gone.