undercut their GoldSource profits
These days the amount they make from Goldsource per year will probably be around what they make from CSGO in five minutes of in-game purchases.
At the end of the day, there's absolutely no motivation for Valve to
care about any of this. They're loving and supportive of Goldsource modders, but why would they actively take steps to allow GS to stop being GS? All these Frankenstein's engines like Xash are just such a pointless endeavour. I get that it's fun to tinker with an old engine, but getting mad at Valve for not caring is daft.
If you want Source features, use Source - otherwise, you're better using an entirely open-source engine like Unity.