Trying to relate to the need here. I can understand the desire for sidecar but a small iPad screen as a primary monitor for a Mac Mini - surely an inexpensive monitor can be found? Perhaps even free from a friend who has an old one that they no longer need?
Sorry for the delayed reply. Fair question; the main reason is that I still prefer certain features in MacOs vs iPadOS. I want to keep the portability of the iPad, as I use it not only for photo viewing/editing but as my screen to watch my shows as I make the kids' school lunches at night, etc.
It's easier for those purposes than a MacBook. If I travel it's perfect, though of course that's all on hold right now. I prefer to organize my photos and videos my own way with my own file structure, using the Files app. However, as far as I know there is no way to view my images in Files consecutively in their folders with a viewer; everything has to be imported into Photos first, organized Apple's way. I don't know if Lightroom would act as a viewer in Files, but I'm not willing to pay the CDN subscription rates anyway. There are viewers that work for MacOS (Lyn, for example), and there are other editing programs for Mac that don't have iPad versions. I also prefer how easy it is to manage external drives with Mac OS, and obviously a Mac mini would give more connection options than the iPad, a Macbook air or pro.
The iPad Pro 12.9 screen is vibrant and clear, a bit better than the 13" MacBook screen. Having a Mac mini would allow me to use the mini with my tv and sound system for videos and music, and for other tasks best suited for a proper computer than an iPad (which for me is still not a full replacement). But everything would be perfect if I could use the iPad as a main screen, giving me all the options I need. Well, I guess I'll go over to the Luna site and see if that is the best option for the new mini.