This eerily reminds me of how Valve Corporation handled the now abandoned Source engine, and it's 2004, 2007, and 2013 mainline public SDK branches. After this experience, I will absolutely not consider using closed engines. The feeling of putting years of work into a project on a closed engine only to run into issues further down the line that cannot be resolved by proprietary roadblocks caused by the engine's company changing direction instills a burn that you cannot heal from and takes away a sense of innocence on a hobbyist platform.
Check out Godot! It Looks very promising. I mess around with MonoGame but will look at switching over once they fix some of the bugs with the newly added c# support.