I disagree with your implication that using Steam on Linux makes it pointless to use Linux; I think that it is always better if you are able to replace some proprietary software in your life with freedom-respecting software even if you may still be using other proprietary software.
I’m not sure if you’re aware of this, but the FSF does actually acknowledge that replacing all of the proprietary software that one uses can be an incredibly disruptive, difficult process, and they encourage users to embark on the journey of complete freedom one step at a time - check out their Freedom Ladder campaign.