I disagree, some very poor people most likely prefer the ads then parting with 2$. One thing people forget about advertising it is a pretty good at price discriminating. Ads to rich people are worth more than ads to poor people so advertisers can make more money from those with more money to spend while still serving everyone.

It's hard enough to get people to switch from whatsapp when signal is free.

Messaging apps, like social media services need your friends to be on the platform for it to be useful. If you start putting friction into that process then the services are instantly not useful. Everyone of your friends that doesn't join makes that thing much less useful for you.

You say "everyone" is willing to pay that. I'll tell people about an amazing app that solves their problems they are complaining about and they go and see it's a couple dollars (one time purchase) and still complain and usually don't buy it.

There will never be a popular messaging service or social network that relies on subscriptions (maybe subscriptions for additional features though)

I think the issue is more if you ask for any amount of money it immediately turns people off.

The process of putting in your creditcard number, entering your password for google play/paypal/etc is just enough of a barrier to go 'whatever!'.

Threema is doing something like that: https://threema.ch/en