And they're washed off during the first machine wash. Your point is?

That and I have lived in SE Asia where there's a lot of handcrafted fabrics, foods, and other unbelievable things checked-in and carried on luggage once you're flying on a plane not bound for a major city. Just look at the Customs line in JFK or Newark to see what they pull out of people's luggage if you're coming from Asia or Africa. I can't speak to other points of origin.

I don't know of any reports of fires in luggage holds.

I know there are fire suppression devices as mandated by the FAA. It used to be just containment, but they changed it to suppression thank goodness.

Nonetheless, I vote for carry-on where you can see it before a single-point failure occurs with the fire suppression system unseen in the luggage hold. I can imagine the fire suppression system either not being charged or armed from my related experience with safety issues in the hospitality industry.