So I just got an email from some marketer with over 1000 addresses in the cc field.

A few of these people have already hit reply-all.

Have people forgotten how email works?

Reminds me of this:
https://reddit.com/r/...

My old company used to do that and it got wild with the replies. Almost funny, if it wasn't so sad.

Someone in my last company accidentally sent an email to everyone. Probably 10s of thousands of recipients. The mail server was so overloaded with everyone replying all (saying "don't reply all" of all things!) that emails were taking hours to get through. It was hilarious