Every negative review of the Ally emphasized windows (and Asus armory crate) as some of the main negatives of the device. Windows gives a worse UI experience, has much higher passive power usage (which prevents you from getting actually good battery life times on low power games like Stardew), and makes things like the deck’s suspend mid game impossible to implement reliability.

You also mentioned that Big Picture mode having most of the features, but it’s missing the QAM and all the nice tools included with that. Asus Armory Crate is supposed to cover some of those, but has had a lot of negative feedback online for not working correctly or having significant downsides like massive deadzones. There’s also a ton of nice features available through decky plugins that are very convenient to use mid game through steamOS.

Not to mention that having windows at all adds to the cost of the device. Average windows license cost for hardware manufacturers is around $50 if I remember right, and they charge more for more powerful hardware. That would be a huge price increase for something like the $400 Steam Deck.

I think SteamOS has a lot to offer, and the only downside (anticheat compatibility) will become a non-issue if steamOS becomes popular enough and companies start targeting it. I really hope to see it available on other devices.