The problem there is the old one all alternative OS have:
Number of Users and availability of programs and their quality.
All openness and security aside, if the people don't get the programs they are used to or need for work/School and the usability needs opening a shell and filling in commands nobody knows what they actually mean and do...you will not enter the market.
That's why Linux never made a true impact in the people's homes. Even when you use Linux just for word processing and browsing-like i do-you will be confronted with the shell depending on your hardware. People aren't used to this anymore, they want graphical user interfaces and convenience.
GrapheneOS knows this and created a truly easy web installer and the easy opportunity to revert this to stock OS. Also they added a way that you can use almost all apps that you might need through PlayStore access.
Valve will have the same problem they had with the first steam deck. It's too expensive and in the end you don't get the same you had with a windows pc-though they did a lot to make games run on Linux distros.
But, to spread this new SteamOS it must offer benefits aside gaming. And the other problem is, why would one need it, if he hasn't a gaming PC.
Gaming PC and also the new steam machine will cost a lot of money . Publishing this during a global recession is counter productive.
The new steam machine is estimated to cost around 800$. There won't be many buyers at this price point and it will soon be forgotten.
And assumed the new OS could be installed anywhere without the gaming aspect, why use it? A Linux Mint or other variants offer security, for the super paranoid a USB live system is the first choice.
I like the idea of having competition, but here I don't see where the consumer benefits? At the moment it's not much more than an expensive gaming device with an own OS that let's you run browser and word processing.
Take the last away and you got an Xbox, take the browser away and you got Nintendo and PlayStation.
I doubt this will amaze the masses and make a change.