There are advantages to soldered RAM. It can be faster without extra cost. The issue is it’s not upgradable. It depends if they’re targeting people who will upgrade their device or people who won’t on which is the smarter option.

If you want SteamOS though, there are already plenty of options. SteamOS is just another distro based on Arch. It isn’t doing anything special (except on the actual Steam Machine which will have extra hardware to, for example, turn on when you press the button on your controller). There is literally no reason you should be waiting for it, especially if you want just a desktop OS that can play games. If you want a console-like experience, where it’s harder to access the desktop and more restricted, then there still isn’t a reason but it is a thing you can do.