Here's the problem with the Steam Deck, and every modern handheld. (And actually also the DSi XL noted in GP.)
The DS Lite can fit in your pocket. Comfortably. Not, like, technically fit, at the cost of making it uncomfortable to walk around. You can keep a DS Lite in your pocket all day, ready to be pulled out the moment you enter the crowded subway, or while in the doctor's office. Situations you didn't necessarily plan for.
The stupid Switch, and especially the even larger Steam Deck, I never actually have with me when I want it. All I have is my stupid iPhone, which is great for many things but doesn't have buttons, and it turns out you kind of need those for games. Developers have been trying, for nearly two decades now, to find ways to avoid buttons, and it even kind of works sometimes, but not really. Not when you want precise control over a character.
No one makes portable consoles anymore. I mean, they're portable in the way that a laptop is portable. They're handheld in the way a vacuum cleaner is handheld. But they're not portable like the DS Lite or GBA SP.
You can buy these little Android things designed for emulators, and I've almost bought them a few times, but I've pretty much already played all the NES games I want to play, and because there are very few new NES games being released, the library doesn't expand.