The idea is that with features like this widespread, the probability of a theft leading to the thief repurposing the stolen device will be lower. Therefore, the incentives for thieves – the ones acting rationally within the paradigm of being a thief, at least – will be lower. Therefore, the number of thefts, generally, may go down. A few big ifs here, of course.

I've always felt that system lock was about preventing theft of the device data, not prevention of flipping the device itself. Like tons of other people, I have sensitive apps and data on my phone (banking, cloud services, etc), but unlike tons of people, I take great care to ensure logouts after I am done accessing said apps or data. Even so, a thief snatches my phone, I don't care what happens to the phone itself, so long as the data is safe.

Knowing the thief does not have access to my email, banking apps, nudes, etc is worth a huge piece of mind.

The phone is worth less to the thief, so theft becomes less lucrative and there is less incentive for the thief to steal your phone.

The cost of your phone might be low or covered by insurance, while all your private information is much more valuable.

I never spend more than 200 euros on a phone and consider them disposable. If I lost my (locked) phone, I'd merely get my (identical) backup phone from home and continue as if nothing happened.

The world runs on incentives. Remove the incentives you remove most of the behaviors.