Speaking for KDAB, only a relatively small fraction of our overall revenue is paid work on open source codebases. We mostly get paid to work _with_ Free Software and to teach it to people. Living off of maintaining or improving Free Software is only sustainable for freelancers or small boutique companies. Even the big foundations don't fund all that much development, they mostly focus on governance, infrastructure and promotion. The Linux kernel is possibly an exception, in terms of feeding an appreciable number of people working on it professionally outside of a company directly selling it.