Threema costs money which is a no go for almost everybody.

And then people complain to be the product...

Not even that most of the time. People just don't really notice or care.

WhatsApp became massive before being bought by Facebook and you had to purchase it for $0.99 (or $2,99 it's been almost a decade so I can't remember the exact price). So no, as long as the network effect is there, costing money is not a no-go.

Except it wasn't really enforced. They kinda made you think you HAD to pay, but extends it to another year even if you didn't pay. So it was actually free.

You literally had to purchase it in the app store on iOS (or pirate it, I guess).

I can only tell you what I have heard from many people myself: 4€ for a chat app is not on.