Proton is the explanation to the steam deck library. Stadia came out 2019 and probably has been in development for a few years prior. Proton is relatively a recent thing.
The issue is in a low latency environment, translating directx to vulkan may have overhead.
Proton is just pre-built Wine with Valve patches. Wine existed for years and DXVK wasn't Valves' own project. They just sponsored some open source project and driver improvements and integrated it into Steam.
I seriously doubt that if you count every single developer that Valve hired to work on it as well as sponsored 3rd-party developers there likely gonna be 25-30 people or less.
I pretty sure that more people worked on Stadia within Google than all people working in Valve on all of their projects.
This is really simple: people no longer trust Google.