I'd imagine that if these parcels are not strapped down and the load shifts in flight because of severe turbulence, that could shift the center of gravity enough to result in a chain of events leading to a crash.

You have to secure the loads or they can shift, causing the airplane to crash. I agree that you take out the seats in the aircraft but I don't know if the cargo "hooks" are certified or even there. Cargo can not be loaded into aircraft if it can not be secured into place.

Seats are installed via a track system present on the floor. Seats are part of what’s called BFE, and a flexible track system means you’re not drilling holes in the floor.

Limited loads could be secured in the passenger cabin using the same tracks. Whether or not that actually makes economic sense is a different discussion.

You can see this in action here: https://www.reddit.com/r/aviation/comments/8scjkk/ever_wonde...

Looks like someone could start making cargo containers that secure into BFE, once the seats are removed...

I seriously thought that the need to strap down or generally secure this stuff, and that it would be done, was so obvious as to be beneath mentioning. It seems not.

How do you properly secure cargo in receptacles intended to secure humans?

German post uses it for next-day letter delivery. They just put the boxes in bags and use the seatbelts to stap the bags to the seats.

Is that really an unsolved problem in airline business? It can't be that hard to build small containers that fit on the seat and can be fastened with the seat belt system.

Obviously, you won't want to do this for general cargo transport, but for "hey, we have a grounded plane, but transporting air mail gives great rates right now" situations?

So it's not a solved problem on massive scale because there aren't frequent and long worldwide air passenger traffic shutdowns.