I haven't heard many people talking about a key group here.
There are quite a few people out there who play the Deck handheld 95% of the time, so they rarely dock it... but they also aren't out and about traveling with it.
They just play it in comfy spots around their house, without ever really hooking it up to a larger screen extendedly. There are probably quite a few of them talking in this thread.
For any of these people wanting a performance upgrade: Steam Machine is going to be HUGE, and much better than switching to a slightly more powerful handheld at nearly the same price point.
These people can hook the SM up to a TV to check on it alone if needed... but primarily they will locally stream from it to their Decks. And it'll absolutely crush 60/70/90 FPS (the common max display rates on the Deck screens, depending on what flavor you have and whether or not you're overclocking the LCD display) at 800p, with graphics cranked WAY up on a ton of games.
It'll definitely be a fantastic era to be a household Deck gamer.
Remember that scene in The Shawshank Redemption where it establishes that the corrupt warden was ruining local businesses because he had no labor overhead costs when deploying prisoners all over town to do project work? And then the local businesses started bribing him to stay away from their livelihoods so that they wouldn't go bankrupt?
Same as modern times: except, it's 10,000 times larger in scale now, and the smaller businesses can no longer afford the bribes. But that's okay, because the government stepped in to pay instead.