My employer (Wikimedia, which is a non-profit) has been running all our server infra on pure open source for many years, for very principled reasons. We even publish all of our internal infrastructure config in public git repos as well. We're an open-source-only shop that's doing very non-hobby work. We have thousands of servers, we run our own CDN on multiple continents, we have somewhere around a billion unique users per month, and our average global rate of inbound HTTP requests is around 135K/second.
However, it keeps getting harder to stick to our open source principles every year as more projects shift towards open-core models with this kind of "Anyone who needs feature X is surely a big corporation making billions who doesn't mind running closed source software" type of thinking. It's not just bad for us, it's bad for the whole larger open source and Internet ecosystems if all the important bits for running at scale are locked up and hidden away in closed-source software.