It is probably easier for Switch 2 to offer enhanced versions than Series X because of the translation layer they made. However, you can't just instantly make everything work perfectly. For example, Yoshi's Crafted World is mostly 2D assets pretending to be 3D that are animated for 30fps. You can't just magically make that 4k at 60fps and if you force it then it just looks bad. Something like enhanced Pikmin 4 would actually be much easier to pull off.
But even for games where you technically could, the UI elements are made for lower resolutions and weren't designed around adaptive scaling, so you'd have to go through and redo all that so you don't end up with microscopic text and alignment bugs. So with a lot of lesser games, you have to wonder if it is worth going back and redoing a bunch of such elements to make it work properly. Yeah the DLSS can manage that on PC but that is because devs originally created the UI there with adaptive scaling in mind.
Now, for games that are a bigger deal, like Luigi's Mansion 3, Splatoon 3, Smash, Xenoblade and such, you'll probably want to do a full on "Switch 2 version" to do it really well for one and then secondly to cover the costs by selling the upgrade. I'm sure we'll see a lot of enhancing patches since it's so damn easy and they may want to use bigger ones as a pull for NSO, but it does take some effort to go back and adjust what isn't just the 3D part.