The maintainer has to pick a side and commit to it, and deal with the downsides. Alternatively, they may choose not to play.
Still, it looks like he maintained the library for a long time. He no doubt has more knowledge about the code base than outsiders. That ought to be valuable to corporations relying on the library and contributing security patches.
Because he continues to work for free? Companies are amoral actors. They aren’t going to donate out of charity and if someone wants to give them free work they won’t say no
Why buy the cow if the milk is free? The license let's them use it without payment, and in a just world, they'd pay all the maintainers of libraries they use, but ours isn't a just world, and we need to formulate our strategies with that in mind.