[the_biot]

> There are now hundreds, if not thousands, of decent editors who have stopped contributing due to toxic, non-productive people.

While this is likely true (though not quantifiable), I would add there is also a pattern I've seen play out where somebody gets really into Wikipedia, starts stablishing turf on some subject, goes a little power mad, then gets knocked back by rules designed to keep such people out of power. Said editor then leaves Wikipedia in a huff, and a prolific contributor is lost -- but it's broadly a good thing.