I think you’re missing the point. It’s not about OS backwards compatibility, it’s user library backwards compatibility. Imagine if proton didn’t exist and you have 15 years of Steam library that has expanded on a yearly basis. You now buy the Steam Deck to play your library. What games can you play? I guarantee you couldn’t play 99% of your library because less than 1% of all games on Steam have been made natively for Linux. If you can’t play 99% of your library what’s the point of owning the deck? This is why Valve is pouring money into Proton, because Proton is the tool that gives users backwards compatibility for their library. Without proton the Steam Deck would be an utter failure.
It’s also why they don’t need to incentivize native builds, because they already solved that problem on their own with Proton. Why put effort into having developers develop native builds when you could just put that effort into Proton and essentially get the same result (and extra benefits) without hoping the developers do something they didn’t want to do in the first place?