Stadia changes the math a bit. If you’re already targeting Stadia, which uses a Linux variant under the hood, the time to make it widely available might be worth it.
Might be... but (I assume; I don't have any specific knowledge) that Stadia runs everything on a single known software stack, and quite possibly a single hardware stack. This would make it an easier platform to target than the wider range of general consumer hardware running Linux.
Personally, I run Linux as my daily driver and I hope game developers see the success of Steam's Linux support and decide that targeting Linux is viable and worth doing. But I wouldn't assume that building for Stadia necessarily has a big overlap with building for consumer Linux.
Valve seem to have been addressing that with their runtimes.