you people are letting Valve make their money on free software.
Making money on free software is absolutely fine. Even GNU, perhaps the most hardcore free software group, has this to say:
You may have paid money to get copies of a free program, or you may have obtained copies at no charge. But regardless of how you got your copies, you always have the freedom to copy and change the software, even to sell copies.
There’s absolutely no problem with Valve profiting from free software, the only obligation is that they share their modifications to any free software they use with their users. Here’s their fork of Proton, and here’s an article about the rest of the software stack they have modified (i.e. they’ve made lots of changes to the graphics stack, as well as various other parts that are important to their use-case.
So it’s not like they’re taking and not giving, it’s a two-way process where they and the community both benefit from using free software.
The other points have been well covered here, I just wanted to point out that making money is not a bad thing.