This framing is just wrong; there has never been, is not currently, and will never be a guarantee that you can sustain a company entirely off of an open-source offering.
If you are concerned about your livelihood then don't hinge it on the viability of open source projects to underpin your business model.
i think the issue is less "sustain a company" and more the unspoken qualifier "extremely valuable".
sustaining a small company offering commercial licenses or hosting or support or consulting or whatever for an open source product is not going to be that much more difficult than sustaining any other kind of small company.
the issue that I see most often are OSS devs not approaching the problem like a company would and startup founders looking to build a unicorn while also keeping some kind of purity wrt open source.
not everything needs VC investment with valuations measured in the tens of millions but everything does need some level of formal business development (even if that business is of the non-profit variety).
Cool, and what is your proposal to people who both believe in open source, build on open source, but would also like to be able to put food on the table and enjoy their work?
Blender and Godot both show one possible way to putting food on the table and spending your time contributing to open source projects that you love. Other companies provide consulting and related services. Business is hard, you can't just write some code, throw it over a wall with an open source license, and expect the money to start pouring in.
You can get a job at a company like Google, Amazon, Microsoft, or hundreds of others where they pay folks to write open source code, too.
If you are concerned about your livelihood then don't hinge it on the viability of open source projects to underpin your business model.
I've said it before, I will say it again.