The ARM/Linux world is somewhat fragmented right now. There's vanilla ARM/Linux/glibc which has no drivers (vanilla Linux cannot use Android drivers). There's Android, which has drivers but has completely different APIs than desktop Linux. Then there's franken-Linux which tries to shim desktopish APIs on top of Android drivers. Which one should Steam target?
It's not like people are installing Linux on Android tablets all over the place and Valve needs to support those users. Valve doesn't need to worry about those devices. They only need to worry about the ones coming out with that Nvidia chip, for which Nvidia will definitely provide the drivers. So what you talked about is completely a non-issue here.
Then there's a question about how many months or years it would take Nvidia to definitely write those drivers. But yeah, if Valve wanted to lead the development of a new ecosystem at any cost I guess they could do it.
