And as it was one of the few messaging platforms to be (in the future) paid at all, I cannot understand why it ever got popular…

Because that way people thought they were directly paying for the service they were using, instead of being the product of said platform, having their personal data harvested and sold to the highest bidder?

Well, sure, Meta cancelled the subscription plans later but to me it sounded a red flag in the first place.

The red flag is to look at a free meal and not wonder what the catch might be. Especially to this day, with all we learned about what the tech majors do with all the data.