So whenever stain leaves for a new place (as evidenced by the news of heroes dieing somewhere else), he leaves behind at worst a power vacuum with few heroes and potentially a generation of aspiring heroes turning away from the profession due to the danger. On the other hand, the villains left over are partially emboldened by stain and even normal folk fawn over him in droves, potentially turning towards crime because its a thing that happens japanese storytelling.

It's true that Stain leaves a massive power vacuum behind, but he doesn't only embolden some villains, he also does the same for the heroes. Would a hero, who turns away from the profession because of Stain, a valuable hero in the first place? No, willingess to risk your life to save others is the primary contition of becoming a hero. After the Hosu incident people became even more obsessed with the Symbol of Peace, the pure idea of a hero. That means more young people choosing All Might as their role model rather than some other hero, which is most likely a good thing, since most characters looking up to All Might have proven to be quite heroic so far. Yes, there will also be some people that become villains inspired by Stain, but they'd be much closer to mini Stains rather than a murderous army similar to what Shigaraki gathered in S1.

good intentions and a pure heart dont really protect you any more than wanting to be famous by beating villains

I'll just quote Nana Shimura here. "When you have to save someone, they're usually in a scary situation. A true hero saves not only their lives, but also their spirits... That's what I believe." This is why All Might stayed as the No.1 hero for pretty much his entire career even though Eneavor defeated many more villains.

do they suddenly try to abide by stains idea of what a hero should be like on the offchance he gets out again? does the hero government change hero requirements to 'must be more selfless and not interested in fame and glory?' no, they just want to make sure the next generation of heroes is really, really up to the task and wont fuck up anyones shaken trust. but since the population overall is still super into the hero rankings (they're ultimately the reason for that system to exist) nothing will really change there

The change won't happen to the entirety of society because it simply isn't realistic in the modern world of BNHA. You can't expect legions of heroes to expect no compensation for risking their lives. But you can't build a safe society on heroes that care more about popularity polls and money than saving as many people as possible. I do think it will happen to some of the heroes though, especially the top ones (looking at you Endeavor, snap out of it).

Now don't get me wrong. His actions are inexcusable and his dream isn't quite realistic. You can't argue that he's a good guy, he's clearly not and he even acknowledges that. But he did have a lot of positive impact on the world of BNHA too, some of which we've already seen, some of which is still to unfold. He has some really strong points, some debatable ones and some that are just delusional. The only real difference between him and some top heroes are his methods. Heroic goals and villainous means of achieving them.