You're right. I was more thinking of "they gain almost nothing by protecting their copyright over the engine" as in having a competitive advantage using their engine as a secret sauce. Because Valve is barely in the game dev industry to begin with and makes most of their money from steam sales. But yes they would have to either support the engine or make the decision to not support it.

>when they inevitably break third party users.

Wouldn't versioned releases solve this issue? You can ship a new release every 3 years (lets say) and developers would expect things to break between versions which is the case for all engines.