It does make iPhones a far less attractive target for theft, though. Much harder to resell a locked device.
It obviously doesn't stop the supply chain of stolen parts but it makes it so legitimate consumers will ask questions and legitimate stores will be less likely to buy stolen phones for parts.
In this case, Apple has forcibly become the gatekeeper for all parts, both used and new, and has plausible reasons for why this is good for the customer. It's really quite brilliant.
Frankly, I'm fine spending an extra $50 if I break something vs depriving the market of stolen iPhones. Like everything, there are costs and benefits.