On the reply all subject, it's vital for any substantively collaborative workplace environment. Sure, some individuals are not very good at selecting who should and shouldn't be on various emails, but that's far outweighed by the benefits.
In terms of the situations where 50, 100, 1000 people are on an email, if configured properly, it's done with groups that are supposed to be controlled so that only certain admins can actually use them (even for replies). We had a situation once in my office where one that wasn't configured properly got out of hand and there were nearly a hundred reply all responses (mostly saying unsubscribe or something if that nature). Eventually the CEO replied saying everyone stop using this email chain - which was also stupid, he should have just had IT disable the email group that was causing the problem. This is at a company with 10k+ employees, probably 2k on the chain.