Obviously Facebook took off once it dropped the 'college student' requirement and opened up to the general population. But once that first generation aged up, the younger generation didn't follow and Facebook became the place your grandma posts Biden conspiracy theories. Widening your target audience can get you an initial boost of users, but you end up competing with every other platform doing the same thing. Then some new platform opens up, all the cool kids go there, and the old platforms gradually get dumber, withers away, and dies.

and yet users will not complain, I heard people say "those who use old.reddit.com are just cranky senile people" or something like that, some of us prefer old.reddit.com even if we were nine when browsing on desktop

You misunderstand what their product is at this point. Their primary product is advertising conversions by site users and visitors, with a minor secondary in user data - actual "content" is a distant third. By removing or restricting the means by which users can avoid interactions with the customer base, they drive traffic to the customer-sponsored content on the site. Yes, what they're doing will reduce organic traffic; that's the point. They would be very happy if every post visible to a user was a paid post.

Reddit is an advertising platform which allows user created content. I expect the majority of the Fediverse to go that way too within two years, regardless of Meta Threads. This is the Internet. This is what it is, what it was always going to be, and how it will be going forward.