The answer is "then a viable fork will sprout up and we can switch to that instead" which is what has happened for every notable widely used OSS project I can think of.

And it's not like you have to sit idly while that's happening. A small company might not be able to fork and maintain their own fork but they can pitch in with other companies by providing some development time, or money or infrastructure to help make an OSS fork successful as a community project.