It is not to hard to make living off open source for a solo or very small team of developers. There are many revenue models around open source and they have been documented elsewhere. They include providing services like hosting (Wordpress.com) to support (RedHat), plugins (SugarCRM) and "Enterprise" editions (GitLab)

But each of the approaches requires that you get serious traction.

The key here is to stay independent of revenue for the longest time as you can. This is only possible if you are willing to work as a solo developer, working part-time till you hit certain traction numbers.

Open source is a hard and long game as a business. On the plus side, it is very satisfying and if you are able to sustain. There are many domains where open source is already winning big (OS, infrastructure, databases, web) and as new areas "fall" to open source, the risk on the closed-source side will also keep on increasing.