Yes, and then went and serialised the parts.

Good.

That helps with two things I don't want: thieves stealing my phone, and unscrupulous repair shops using counterfeit parts.

That's to make the phones less of a theft target (for parting out). It's a tradeoff, but one most people prefer.

If the manufacturer makes parts easily available, wouldn't the incentive to steal devices to sell for parts more or less disappear? Same thing with counterfeit parts. The market for stolen/fake parts exists because OEM parts are essentially impossible to acquire legally.

Serialisation is simply another way for Apple to maintain control. You can now repair a device, but only with Apple's blessing and knowledge. The privacy/safety stories they sell are just that.

An iPhone-quality OLED display is always going to cost $150+ new as part. That alone is enough value to steal a phone for parts.

Personally, serialization that alerts the user "you have unauthorized parts, if your battery catches on fire or your screen has crappy colors it's not our fault" -> this is good, I am in favor of this, serialization that disables the device or features -> bad.

The part would have to be easily available and cheap.

The disposability allegation was always bullcrap. iPhones have lead in terms of service lifetime, second hand value retention, and manufacturer software support for so long now and by such a huge margin I wonder at the motives of anyone still saying this.