Well I did some math, I added up the percentage of all video cards from the latest Steam hardware survey and roughly 67% of users have a GTX 1050 or better, so for most people a Steam Deck would be a sidegrade or a downgrade. Obviously 33% of Steam's userbase is nothing to scoff at but I don't know, it seems strange to think of Steam Deck as an upgrade. Especially because there's no guarantee of future AAA performance. Unless the thinking is that Steam Deck at 800p would perform better proportionally than, say, a gtx 1060 at 1080p? Would it?
Well, with more access to the data you can probably see exactly where the Steam Deck would be an upgrade, like older 4 cores, 8 GB and at 1080p resolution, so Valve can see things we don't.
I was thinking more about expanding the PC market before, but actually giving a avenue for people with older machines but still active users makes a lot of sense. They will now have an option to update for lot less than before, and that's including probably a
way better experience compared to how Windows is running on their machines, while at same time still being able to play whatever games they are currently playing. And all of this in a mobile format.
Even if that's something like "only" 10-20% of the active userbase, I'm guessing after paying for the Deck they are very likely to buy several games from the suddenly vastly expanded catalog they can run.
Between this market, people just wanting portable gaming, and actually expanding the PC market, Valve will probably sell all they can make for for a while... assuming they can nail the software side.