Imagine you build it with a team of 5 software devs for each of linux, mac, windows, android, ios, web. You build it in Europe paying a generous salary of $80k. You need 1 server per million users (as whatsapp did pre-acquisition), hosted on say AWS. If you get 10% of the world to use it, the total running costs are 1.2 cents per user per year.
That can totally be paid for by sending each user 1 sponsored chat message per decade. That's a pretty acceptable level of advertising I'd say.
It's incompatible with VC's demanding hockey-stick revenue growth though...
Optimistic and dangerous. You will need at least a visionary person like Moxy willing to dedicate his/her life to the project. You need a master mind to get all the security architecture right. BTW You will need an operation team and multiple servers to make sure that everything runs stable. Just look the costs for running COVID-19 apps. Development and operation. Million of Euros. You can expect similar costs to run a service able to compete with WhatsApp.