Wine and DXVK are already running on Android and they play Windows games with the rendering and computational complexity of Fallout 4 at playable framerates on many of the latest smartphone SoCs. It's still WIP, but it's already gone beyond proof of concept, people are using them. Valve don't need the developers to be on-board in order to run their games on anything else, that's why Proton exists.

What Valve want is the dissolution between platform/architecture and store. By my eye, it's the driving force of their efforts, more so than them selling hardware or being the open source good guys. Not to undervalue their work in helping make Linux a first class citizen for gaming, but the core of their business model is getting people to engage with their store, full stop, and being able to sell their games on Android (and elsewhere) would massively extend their reach.

This may go both ways too, there's also been indications that Valve have been tinkering with Waydroid, meaning Steam could also become a store for Android-native games.