It's better value for money for both the gamers and the devs if the devs just choose to engage with valve and get their game running perfectly under proton.
A source port that is optimized as lovingly as its Windows counterpart will probably be faster than the Windows version running via Proton, but the incentives aren't generally there to justify the costs/difficulties. Maybe some day it will be! That would be wonderful.
But until then, Proton seems like an increasingly compelling option for these compatibility layer-based ports of Windows games.
Your latest AAA open world RPG on the other hand? Yeah, you're probably going to have better luck in Proton even if it gets a native Linux port.
Even better would be to compile for linux, but use DXVK-Native (https://github.com/doitsujin/dxvk#dxvk-native) if you think migrating from DirectX to Vulkan requires too much effort.
I think the reimplementations of Windows APIs in Linux, even though alternative to the original, should have similar bottlenecks and edge-cases. So the extra QA on Windows helps the Proton version more.