No, I'm not surprised at all how many people play or pour money into Candy Crush or other mobile games. I used to have a co-worker that thought it was weird that I would bring my Nintendo Switch to work to play on lunch break and that I still bought consoles and games at my age. Yet, there he would be on his work computer during down time playing Candy Crush. The extremely important thing to bear in mind regarding mobile games is that their userbase already own the device required to play them. Nobody buys a mobile phone with the intention of using it primarily to play Candy Crush or Coin Master or Clash of Clans, etc. They already have the phone, and that start downloading apps for time wasters, that for some can develop into wallet drainers. But, regardless of the whales that each of those games accumulates, none of those people are in the market to purchase a $300 or more dedicated gaming console or handheld. It's that type of misaligned thinking that gets someone like Tommy Tallarico to claim the Intellivision Amico had "limitless potential" because it would "tap into the mobile market of nearly 2 billion people" (nevermind the fact that the Amico wasn't even a mobile/handheld device...). Mobile gaming exists completely separate from console and dedicated handheld gaming, because it caters to a userbase of people who already own the required hardware and are mostly looking for something cheap (or so they think until those microtransactions pile up) to waste their time on regardless of gameplay/story/visuals that other gaming media are built around.
On topic, I sometimes think that I might one day like to have a Steam Deck. But, for me personally, it's not as advantageous as it sounds on the surface. The games that I play on PC are generally turn based and real time grand strategy games not found on home consoles. And the more I think about it, I don't think I would enjoy playing that type of a game on a handheld screen. And, the size of the Steam Deck most certainly is a bigger issue for me in terms of portability compared to the Switch. I went to the movies yesterday with my wife, and I was able to carry my Switch, without any carrying case, as it slid right into the side pocket of the tracksuit pants (not sure if that's the correct pants style name) I was wearing. I'm not saying I could carry it in jeans or khakis of course, but it does fit just fine in the style of pants that I often do wear for casual attire. For smaller pockets, I sometimes detach the joycons, and in cooler weather it slides right into the inner pocket that all of my jackets have. So for me specifically, that is a plus in the Switch's favor that would not apply to the Steam Deck.
For the first paragraph: true. And that is also what I mean with value for people personally. For mobile gamers the value of the device they own is (mostly) not made up by it's ability to play games, the value lies in their other features. Gaming is an added bonus, which increases the value perception of the device, but which doesn't make it up completely like dedicated gaming devices.
On the last paragraph: Interestingly. For me the Switch is already too big to do that. But it supports my point, that while the Switch isn't as portable as a 3DS it still is closer to good portability for most people, than the Deck.
And yeah, I also did consider the Deck. But I end up in my mind always at the point that I don't see me to ever play it on commute or so and use the computer to play the games I would want to play on it. So while the device is sexy from a pure technical standpoint, I don't see me use it much.