That's my whole point. Carrier locking used to be standard, even on iPhones, and phone theft was very common too. Carrier locking wasn't a theft deterrent.
Firmware locking is a great theft deterrent. A stolen iPhone is basically just a few cheaper used parts, and all the expensive ones are useless.
One is extremely effective. One is marginally effective. But that still means both of them are effective just to different degrees.
The only theft it is preventing is other carriers stealing customers from one another.
I am arguing that the reduction in theft due to carrier locks is 0. You are arguing that it is small (marginal).
I am saying that no matter what, the actual cost of the carrier lock to the consumer (as demonstrated by used phone values on eBay), is far higher than whatever marginal benefit you are arguing for.
It is a massive net loss to the consumer, and the only guaranteed beneficiary is the wireless carrier.
In theory there may be a point you’re describing, but in SF for instance people just started looting cars instead of mugging (or snatch and grabbing) phones.