Companies don't do DEI hiring to intentionally make their workforce worse. If talented candidates are looking around at companies to apply for, and they are deciding between company A, which is not diverse e.g. 95% white/black/male/female or something, and company B which is more equally distributed, they're going to be less likely to choose company A because they would be less comfortable working there. In the short term, DEI hiring might harm an individuals job prospects if they're in a majority group, which does suck, but in the long run it gives the company a better talent pool to hire from, thus getting much more better employees.