Actually, there may be even higher graduation rates among URM students in STEM fields - this is the claim of some recent research [1]. URM students have lower persistence rates[2] in STEM fields at most universities. Some claim this is due to racism, but the data shows that the persistence rates is actually the same for all races when normalized for academic preparedness. A student with 700s on their SATs is just as likely to persist regardless of white, Asian, Black, etc. Students with 600s and 500s are less likely, again regardless of race. It's just that there's a lot more URM students attending with lower scores.

The expected consequence of eliminating race-based affirmative action is that URM students will instead attend less selective universities where they're more likely to persist. Sure, the university they graduate from may not be as prestigious. But on the flipside, they'll be more likely to graduate in the field they actually wanted to study. They'll also probably save a lot of money, since changing majors partway through usually means that these students take 5 or 6 years to graduate.

1. https://www.nber.org/system/files/working_papers/w14885/w148...

2. "Persistence rate" means the rate at which students complete their declared major. It's rare for students to drop out of university entirely, much more common is that they change to a less demanding major - that is, they don't persist in their initial major.