OP is somewhat correct, but still “short-sighted” with a misleading conclusion. All these valid downsides should be mentioned, but as always there are pros and cons to everything, and in Valve’s case, the pros still outweigh the cons, and you always have to weigh pros and cons against each other.
Valve has done a lot in the last ~10 years to push desktop Linux for mainstream gaming viability and several other features as well (open source shader compiler, Direct3D-to-Vulkan translation stuff, HDR support in KDE Plasma, lots of improvements for the open source AMD GPU drivers, and much more stuff). You can’t simply disregard that. Sure, there are lots of companies involved in improving Linux - but it’s mostly for the server side or the enterprise desktop segment. Almost no big company invests meaningful amount of resources into improving the common Linux desktop significantly and challenging Windows’ dominance for home entertainment/gaming, read: the casual home user. Valve did just that, of course also mostly for their own reasons, but their own reasons still do benefit general desktop Linux massively, and they are almost alone in doing so. And I probably don’t have to mention that having a rich company investing lots of money into pushing stuff does really help development speed. The development pace of the Linux kernel for example is only so high because many big corps spend developers and resources on it to improve it for their own data center use cases. Almost no one (again, except Valve) pours any significant amount of resources/devs into the desktop Linux ecosystem and drivers so far.
Look at GOG - in theory a shining example of how to do several things better than Valve (no DRM, etc.), but they still do close to nothing for desktop Linux, probably because they lack the resources or see it as a wasted effort overall. Like many companies do – the typical chicken-egg-problem. Linux won’t be better supported by companies until its market share grows, but its market share won’t grow until it is better supported by companies. The GOG Galaxy client probably still has no Linux version. That’s just how things have been for a long time and I’m glad to have Valve really be serious about it and demonstrate it publicly that this can work and that this is an example for other companies to also look at it. Their exact reasons or methods don’t even matter - we need companies pushing desktop Linux, or otherwise you can still sit in a corner and cry about Windows’ dominance in 2050 still because nothing really changes on a fundamental level fast enough. Which is why I see it as important to be favorable to Valve for doing this when no one else is doing it. If you want things to change, then do support changes that meaningfully contribute to Windows losing exclusive market share in certain areas like gaming, and tons of people will migrate away from Windows over time because they will start seeing Linux as a viable, practical alternative, not just a theoretical thing. Sure, always be mindful of any disadvantages. But please don’t act as if there weren’t any major advantages as well.
Be glad for how things are developing currently. It could always be better, sure. But it could also be massively worse. And it has been massively worse for a long time. It’s high time to change, and desktop Linux needs all the help it can get to become mainstream. It’s on its way there, thankfully, but that way hasn’t been so clear all the time. Desktop Linux share has always been sub-1% for many, many years. Only very recently it made significant strides forward.