Not great for a chat app, which needs ongoing active servers and someone to fix stuff that breaks, even if you feature-freeze it.

Pretty great for a chat app used by a few billion people, a few $billion is enough to keep things running for many decades. e.g. banks do exactly this, with much more critical and complicated infra.

How do banks do this?

Don't banks also charge a monthly fee?

That works for software, but not as well for services like YouTube

Even for software practice has shown few are actually willing to pay hundreds to thousands for a lifetime license. And you still need to purchase service packs, etc