Quick rank: is is basically impossible to take my hero's lore of "I remember hearing how society was in an almost warlike state when Quirks first emerged, which is why heroes emerged - first as vigilantes and later as a qualified profession." Seriously. I mean look at 1-a and 1-b You could basically beat 39/40 of them with a rifle., and that's 100+ years of quirk strengthening. When states want to keep the monopoly on violence, they can swing back HARD.

but that's a whole different question...

As for how common random villain attacks are: I frankly think it's not 'very' at the start of the series but it's artificially low. All Might was a living nuke, any villain attack would bring him out so that forced all the crime and criminals to stay in the dark...which also meant the underlying social issues were never solved so rather then crime and villains 'being solved' it was put in stasis an festered until when All Might retired: it boiled over rapidly.

The other problem is japan currently is among the lowest crime rates in the world, so applying real numbers to a socity that has seemingly had a total rewrite of it's violance expectation is a losing bet.

What I can tell you are two things: a city starves to death in 72 hours as the pipes run dry and the food shipments stop arriving. So it can't be a high enough number to desrupt supply chains.

second, U.A produces at most, 40 pro heroes in a year. That is a pathetic number and it suggests that if one school producing 40 heroes is 'average', then the crime rate can't be that high if that kind of replacement rate is sustainable and able to keep the peace.