On first point: It’s understandable if the stuff Valve built out was just small library shims to account for functionality Windows games expect. If it’s still designed for gaming and doesn’t impact the user experience, I don’t see that as having the same negatives as Windows addons.
Second one has rung true with my use of the Deck. There are frame spikes and input stalls on Windows machines that, while rare and hard to account for, I don’t notice on the Deck - often making sense only in relation to some background service that isn’t respecting gaming timing.
Third one makes sense to me. Many people on Windows have hit annoyances where they fat-finger the Windows key, and end up in the wrong window while focusing on a game. Having a complete OS boot where the UI focuses on two things: Picking a game, and playing a game, does cut a lot of complications. On Windows, the game overlays on top of the extensive desktop environment, leading to necessary game options around fullscreen mode.
The drivers point I agree with you on; Linux won’t necessarily be easier, especially when it comes to peripherals.
The last point about modification is still going to be true if Lenovo puts out a Linux handheld; in fact it’s a long-understood benefit of Android smartphones too. I like my iPhone but I’m certainly locked off from things like alternate app stores.