Yup. I didn't use Steam until they came to Linux, and I don't have any loyalty to them. I'll buy from any platform that supports my OS of choice.
If I used Windows, I would probably use GOG because I value DRM-free games. But I desire convenience more, and GOG isn't as convenient as Steam on Linux, so I don't. It's pretty simple.
Even if GoG's launcher was on Linux (which BTW last I checked was THE most requested feature) I would still buy on Steam because it's not only that Valve is releasing for Linux, they're also investing money to finance Proton development, so they're actively spending money to make Linux gaming experience better for everyone, which is why they'll get my money over any other company, especially one that doesn't even support the OS at all.
I don't buy based on where the company chooses to invest its money, I buy based on who has the best overall experience. With GOG, I know I'm getting DRM-free games, and that has value. It's not enough value to jump through hoops over, but it's enough that, at the same price, I'll prefer it.
GOG doesn't have every game due to that policy, and it'll probably never have as good of an experience on my Steam Deck, so I'll still buy from Steam, but GOG would be my first place to go.
But since GOG doesn't even support Linux with their client, I don't buy from them. I will only buy if they provide good support, and they don't.
That's not exactly true. Usually that only applies to MP when connecting to the hosted servers, not not when running locally or on self-hosted servers. That said, that should definitely be listed on the store page.
That's lame. Fortunately most of those are merely cosmetics or rewards for doing something online, but that should still be called out on the store page.