I don't understand what they got basing off the Source engine. Maybe it made sense when they started 6 years ago - to allow using Hammer and such. But at this point they've made their own editor, networking, scene system... why is it still attached to a giant legacy codebase.
As a game dev it surprises me the constant push from some Linux players to still make a native Linux version, considering how good Valve's Proton layer is. I don't think they understand that while making a new Linux build can be trivial (in Unity), dealing with all the support issues for that isn't. I don't have time to track down audio issues for some random Linux version for 0.001% of players
Yet if I say I'll only support Proton on Linux I am inviting myself to constant hate mail