Such a high percentage of games these days are indies that require no horsepower, or quasi-indies that only require a little bit of power, the Switch does all these things perfectly. In fact using a 200+watt device to play something like Hades feels stupid, and maybe even a little bit unethical if we are all starting to track our carbon footprint.
So I'm happy for the Switches success, and the relative success of the Steam Deck. It feels like a step in the right direction.
That said, the Switch is 2015 era tech, that's when the iPhone...6S/7 came out. Switch feels a bit lacklustre doing anything demanding (including even opening the e-store). It's starting to show it's age and specs. Once Nintendo release the Switch "Pro" or whatever they will have shored up the only weakness in their 100+million device and ecosystem. But until then there is a bit of a gap in the market for a higher performing mobile device (outside of the obvious answer, which is mobile phones and tablets).
The Steam Deck, more than twice the size of the switch in volume, more than twice the watts, 1.6X the weight. But I would rather the out and out appliance, designed from the ground up to be what it is, than this - trying to kludge a PC down into mobile with an existing non-mobile ecosystem. Seems like the tail wagging the dog a bit. And if legacy ecosystems were not a factor there is no way anyone would choose this way of going about it.
That said, Valve did a good job for what it is. I thought it would sell 100k units to nerds and die off. It seems to be holding up at well over a million units so far. It must be a good experience for a majority of users.