Returns and refunds happened because the EU warned them that if they didn't, they'd create legislation so they'd have to, as they were already in a grey area under EU law. EA had a similar refund policy for games bought through Origin before Steam did.

Other than that, nearly everything you listed was done because it made business sense and would lead to more profit. Decoupling PC gaming from Windows by working on Linux means they're not at the mercy of Microsoft's whims, and was started at a time where it looked like Microsoft might make a version of Windows that could only install third-party software through the Windows Store. Discouraging kernel-level anti cheat discourages one of the last hurdles to Linux being able to play all Windows games. Supporting VR lets them still VR games through Steam.

Not being publicly traded lets them be concerned about their long-term profits above their short-term ones, so they won't do things that tarnish their reputation nearly as often as their competitors, and can do multi-year projects. They look good because their competitors are bad rather than because what they do is altruistic.

Edit: I just did some maths. The Steam Deck has sold somewhere around 5 million units and a Windows licence costs somewhere around $50 to an OEM with a volume licensing deal. Both these figures are approximate as I couldn't find precise numbers, but they're enough for a ballpark figure. This means that Valve have saved around $250 million by shipping the Deck with SteamOS instead of Windows. Even if my figures are way off, it's still a huge amount of money and goes a long way towards making all their work on Linux pay for itself.

single handedly making linux a viable gaming platform for the casual user

Well, not really single-handedly. Most of the work was done by open source communities, Valve's part was mostly in integrating it and normalizing it, making it easy for users and giving developers an incentive to support it. They did their part and I love them for it, but I'm also grateful for all the volunteer contributions to projects like wine and dxvk!