Steam Deck

I hope so, but only if the option to install other OS's remains an easy option. I love android but installing a different operating system on my phone is so much of a pain in the dick that it's not even worth it.

I feel like I'd probably avoid a handheld if the option to install windows wasn't there, even if I don't end up using it much.

If my choice was a default windows install with the option to install steamOS myself, or a default steamOS install with no other options, I'm choosing the windows install every time.

Looking at how many celebrated ROG Ally shipping with windows I doubt it will catch on. Only possibility I see is if valve would do profit sharing with the handheld maker for purchases made in the steam store.

For a third party to ship with steam os now would essentially mean they are also supporting the largest player in the market with no gain for themselves.

I'd be OK with that, it's been excellent so far!

I do not get reference to smartphones. In US iOS is dominated system.

No. Steamos is only really great on deck because of the whole making the hardware and software thing. If other people use it it loses that and you end up with a computer with a less compatible OS.