If only a couple of percent of your sales come from Linux though, the development and support efforts can easily eat that up.
Ding... you just hit on the exact reason Valve spends money developing Linux things.
The point of burning the money is so that game developers need not.
If you develop your game right now in say Unreal Engine... or Unity or any of the major engines, they are cross platform. You don't have to make major changes to your game to publish it on MacOS/IOS/Android/Linux. If you used DX code mainly you are going to have to do porting which yes is more expensive and tends to be a pain in the backside. If you however used Vulcan code... and a major engine. With very very little work you can easily release your title on 5-6 platforms instead of one.
That is what Valve has been spending money on. If as you say windows ever gets locked down... boom Linux is ready to go. If that doesn't happen and Macs see a surge in gaming use, same thing the industry can shift that way and Valve will profit. People also forget that there is a steam mobile.... all the tech they are developing to ensure the industry is cross platform and not locked to stupid DX also helps steam in that market. The day isn't here yet but 5-10 years from now its very possible that the same games we play on our PCs will be running on our phones and tablets... if that day comes Valve will be set to profit either way thanks to spending money on open APIs ect today.