Otherwise, what use case is worth 3.5k? It's not a work device, because the sidecar is limited to a single display. It's not compatible with OpenXR, so it's not a simulation device.
1. You need to have a lot of control over the room lighting. If you can't block out all the light the image quality suffers a lot.
2. You need to have a clear wall or projector screen, which might mean giving up some useful space in your home.
3. The black levels are awful compared to OLED.
As for guests, other than the occasional movie nights, we usually play a board game or are making something as our activity.
I have my doubts about productivity too. One or two displays doesn't really make the most of it and you still need a Mac to run serious computing (better than iOS or iPad OS) apps.
I also wonder how well the eye tracking mouse pointer plus finger pinching for clicking will be when doing this for real work. I'd imagine it fatigues quickly. Oculus uses hand movements plus finger pinching and that's way worse for comfort than using the VR controllers.
And of course there's the elephant in the room, the price tag.