Does it, tho? Wizards are powerful, sure, but they’re severely outnumbered, and almost completely ignorant of technology. What’s your Avada Kedavra, that moves slow enough that untrained school children can dodge it, gonna do against the Mother of all Bombs? Or a nuke?
Don’t underestimate muggles.
Yes, strongly. Lead exposure, especially in childhood, is known to lower IQ and also decrease empathy.
The other things you mentioned are definitely major factors as well, but yeah all that lead fucked the older generations brains.