Spam was terrible before CAPTCHA. Every forum software had a half dozen different anti-spam plugins that only supported that specific release and were incapable of catching anything novel, so you still needed a team of moderators covering every time zone to handle all the crap that made it past those filters. Unmoderated forums turned into bots endlessly replying to each other in a matter of days.

CAPTCHA did a ton to reduce the amount of bots successfully signing up, and it took years for them to adapt. Captchas turning into a tool to make you create free training data for self-driving AI sucks, but capitalism is gonna capitalism. The captcha variant that was used to help digitize old books was pretty cool.