"supported by grants and donations" according to their site.

Why is everyone doing business with a black and white strategy? On one extreme, everything is free and funded by selling user information and on the other extreme, everthing is free but funded by donation. What is wrong with 2$/year model? Everyone (I mean truly everyone) is willing to pay that.

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)

Would agree with your sentiment, but Whatsapp itself is an example of a messaging service that relied on their 1$/year model and had around 200 million active users before it went free.

I wouldn't say they "relied" on it. It was not enforced at all (lots of people used it without paying $1) and they sold out to Facebook. It would have been interesting to see how things played out if they had stayed independent.

They started charging after they had achieved scale.

No, it cost 1$ from the very beginning on iOS.

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.

WhatsApp's $1/year charge only applied in certain (rich) countries.

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

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!'.

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