As you're probably already aware, it's a computer! Not like your phone is a computer, or another game console is a computer, you can actually do whatever you want with it, even install windows (wouldn't recommend it but you can), or android, or another distro, without having to do any inane rituals to unlock this or jailbreak that. There are already several linux distros made for steam deck. It even lets me mount my NFS share without any hassle. As a mobile device it's very friendly, if a bit bulky. With the trackpad you can actually use a desktop-oriented UI or mouse-and-keyboard game as easily as you would on a laptop.

As for steam OS, even in the gaming mode it doesn't feel limiting. You can run multiple games/programs and switch between them. You can use your usual music player program in the background while playing a game, or firefox to look things up. There are community controller layouts that you can download easily. Even minecraft java had some very sane options that make playing it without a mouse and keyboard not a terrible experience. With emudeck you can play switch games like they're native and they even show up in your steam library.

In the "desktop mode" it's just KDE. Nothing is imposed on you. It isn't corporate or opinionated, and with a dock it's literally just a regular computer, but you can take whatever you're doing with you! Being immutable it forces you to use flatpaks which I'm not a huge fan of but they're alright. They haven't been enough of a problem for me to install something else. The touchpad keyboard thing is actually really efficient.

I think it helps linux adoption quite a lot. It isn't trying to "fix" linux or impose their interests on it like other companies are doing, it just builds on top of it. What is the point of getting everyone on a linux which has become an "ecosystem", where even if your software freedoms are on the face respected, any modifications which are out of the "ordinary" cause opinionated software which expects other programs to behave in a certain way to stop working? All that to say, as of now valve is not doing this.

I don't have any opinions about Valve outside of this. I don't even play many video games, I just bought it to use as a neat computer!