No, they wouldn't. This thinking is what has lead the world where it is.
Say you have 2 billion (10^9) users paying $1 per year. You follow inflation. That's 2 billion a year. That covers a lot of workforce, and a lot of servers.
This idea of infinite growth boggles me - how can people with math background insist on such impossible thought?
I'm unable to find the exact quote but soon after the WhatsApp acquisition, when the $1 fee was waived there was a message that went along the lines of "imagine how much more money WhatsApp could make". If someone has it, please send a link.
Unfortunately, the world turned towards 1) freemium, and 2) 'consumer as a product', and it will not be easy to roll back the free chat. But I think it is possible. Make signal survive for 2 years based on donations, or this 1 buck a year/month, and we may be able to have a long term alternative to whatsapp.
BTW, they had just 50 engineers when FB bought them with 450M users (+1M/day) and they supported more devices back then. Just with those costs and the network effect, keeping the subscription model would have meant a solid source of honest revenue.
5% of that is 50 million.
For Christ's sake NO!
FB never charged for whatsapp for a good reason.