This is a straw man argument. The problem is not closed source but lack of regulation and power asymmetry between the large company and a small customer. These types of bait and switch one-sided contract changes should be illegal. In fact, the way how Unity approached this is likely illegal, since they have retroactively changed the contract while attempting to hide this fact from the customers.
We need to strive towards establishment of fair, enforceable legal frameworks that prevent this kind of abuse from large players, instead of magically hoping that open source will fix everything.
I agree completely, but open source alternatives like Godot make it easier for people to pack up their things and leave, and you'll never need to deal with this issue if your business uses open source software. Open source can be a part of how we hold abusive corporations to account, and it's a way for developers to take action without hiring a lawyer.
We need to strive towards establishment of fair, enforceable legal frameworks that prevent this kind of abuse from large players, instead of magically hoping that open source will fix everything.