I know - and there are some nice howtos to even use it as an USB and ethernet hub.If you dont mind taking it apart and installing a display driver board in place of the motherboard, this can be done. I’m not ready yet with my 2017 model but maybe at some point. I also agree with your statement. It would be great to have a secondary mode that allowed it to be used as a monitor when not used as an iMac and have both available. I would buy into that for sure.
But this would just be a hobby or ecological sacrifice - because alone the board cost about 600 EUR these days on Aliexpress, so even if you don't count your own time it is economically a limit case.