I agree with a lot of the points here, the fact is the 3DS has a banging library (especially with the DS backwards compatibility), doesn't emulate well, and it has arguably the most attractive form-factor of any handheld. It's way more compact than modern handhelds like the Switch, and with the clamshell design you don't even need a case for them, just shove it in your pocket!
Another thing that hasn't been mentioned yet, however, is the fact that they wear out over time. If you use your 3DS a lot, the circle pads get worn, the shoulder buttons go fucky, and many of the 3DS colors have paint which chips off over time. I'd imagine a lot of hardcore 3DS fans aware of these issues will have at least one or two spares ready for when they want to swap out their main unit, so there will basically always be demand for fresh systems even from people who already have one.