Surely this is the advantage of 3D printing - set the print up so everything is in one run and off you go...
Main problem becomes print time but you have all year to plan and produce even if you do not sub-contract the printing.
The suggestion I was responding to was specifically companies like Victrix or Northstar doing specifically hard plastic advent calendars.
If you're suggesting 3D printed advent calendars instead, then you run into the same concerns of a non-standard material for non-printing hobbists, and for printing hobbists - why would I buy a box of printed minis when I bought a printer of my own? This of course leads to the question of, why not do a digital-release STL advent calendar for 3D-printing hobbists? To which the answer is: two of the creators I support on Patreon are already doing that right now.