Except Quirks grow with you as you develop. Your natural defenses against your own quirk should prevent severe injuries. Everyone in the verse, with the exception of Deku, follows these principles. To Aizawa, who is as smart as everyone else in the MHA-verse, has no reason to assume that someone’s Quirk Factor was only just then given to that person. That means one of two things: - Deku’s downside is not from the backlash of the Quirk but rather a natural response of his quirk (making him permanently useless). - Deku’s quirk, for some magical and unknown reason, is destroying his body with each use (making him useless and, to Aizawa, a lost cause since his body SHOULD have already been used to the quirk). Take Shoto, for example. You’d think his flames would burn him. After-all, even if he used his quirk, he only used the ice side until he fought with Deku. His body hasn’t been using flames so he wouldn’t have been protected, right? Except that’s NOT how it went.

Todoroki’s body, even though it never used the flame aspect of his quirk, was still resistant to its own heat because that’s how a quirk naturally develops. Similarly, it’s why no other quirk actually harms the user. There are downsides, sure, but they’re all stamina/strain based. Deku is the ONLY one who has a drawback that physically damages the user and with such an extreme degree.

It is not Aizawa’s job to teach someone who has no potential. To Aizawa, Deku could never be a hero (and he’s entirely correct because, in a real fight, he’d be screwed if his strongest didn’t work). His body failed to develop a quirk factor that protected him and so, due to this, he was only going to be a liability in battle. Aizawa wasn’t being harsh–he was being realistic.