I hope the Steam Deck does turn out to be good for those who want it, but let's not oversell it. The "100% catalogue compatibility" is nonsense for several reasons:-
1. No Freesync means either stutter or tearing because even a stable "locked in" 30fps is unlikely on a low powered 15w (shared between CPU & GPU) chip. Merely looking at Youtube vids of how 15w APU's perform (eg, 4500U), there's going to be a LOT more sporadic frame-rate drops all the way down to 15-25fps even on lowest possible settings (vs 65w APU's that are already 720p/30 limited in the newest titles).
2. Steam certainly doesn't have 100% native Linux support and plenty of "Protoned" games are rated less than Platinum. Many have increased "glitchiness" vs Windows versions whilst others need community mods to function. Eg, Thief 2 is rated Silver on ProtonDB because it works well but needs a community mod (TFix) to do so. How are you going to add this on the device? How to add "annoyance removal" mods to Fallout, Morrowind, Oblivion, Skyrim, or WAD files to Doom 1-2 or are you strictly limited to "vanilla"? Can you mod any game at all? What about the performance drop seen in SteamOS vs Windows due to lack of driver optimisations on Linux?
3. Many older games that "technically run" in terms of horsepower are obviously keyboard + mouse centric, lack controller support or will have non-scaling UI's where UI elements become unusably small on tiny 7" screens. Eg, Doom 1-2 plays very well via excellent native Linux source port (GZDoom) but significantly worse as to how its sold and packaged by default (a DOSBox wrapped title complete with no mouse-look or controller support, can't look up or down, etc...), so you have to add it in (as you do with Windows too). But how on this device? Then there's Dragon Age Origins, which works well via Proton on a large monitor but has a non-scaling UI and is definitely keyb + mouse centric so toolbar buttons are absolutely tiny (unusably small) on a 7" screen unless you install mods like FtG UI. So how will games like these work on the device? What about titles that have only partial controller support (in-game controls but not in menu's so you can't select "New Game" with only a controller present...)? Or the thousands of pre-2001 PC titles with no controller support at all? Claiming the "entire Steam library runs well" doesn't pass the smell test at all when 7" Windows tablets have been around for years that could also run Steam yet despite many games being able to technically "run" on it, the device isn't pleasant to play on if they were designed for keyb + mouse and no controller, and all you have is the exact opposite.
What I've really been interested in is a modern 10" Netbook (like those old EeePC's) that were even smaller than Ultra-Portables but priced far more like Chromebooks, but were fully moddable, had upgradable full sized storage drives and if you want a portable controller you can pair an ultra-compact pocket sized BlueTooth one like the SN30 Pro. As someone who plays a lot of classic games, unmoddable "controller only" handheld computers boasting "100% library compatibility" is an obviously false claim though.