As Game developer myself, and unity user, I was concerned about the new pricing model, however, actually reading the actual changes[1], you see that 99% of the developers are not affected, and in some cases, for those affected, still cheaper to go with Unity than with Unreal: There is a video explaining it well https://www.youtube.com/watch?v=ENoVL68z9PU

For sure an open source solution would be more than welcome, but sounds like impossible, because either those out there are half-backed, or are just finding a way to monetize it too..

[1] https://unity.com/pricing-updates