The resources required to replace the firmware on a non-crippled Android device is access to the Internet from that device. This is not a big ask. Most people who own an iPhone have a data plan and/or access to WiFi.

Sure, only a small minority of device owners can actually make new firmware, but fortunately only one of them needs to make and distribute some firmware for everyone to be able to install it, if the device is open.

Are you saying this as someone who has helped non-hobbyists install new firmware onto their phones? I haven't, but helped many people install desktop linux around 2008-2009 and was often surprised by the problems they ran into.

Most people don't have the resources, knowledge, or desire to replace their car's engine parts. Does that mean it's ok to sell a car with a locked hood, that requires a manufacturer-controlled key before the engine can be serviced?

No, even if they personally don't want to touch the engine, they turn to knowledgeable friends/family or pay a professional mechanic to do the work.

This unfortunately-common view that most people should never even have the option to do anything technical is insulting and maybe even monopolistic (it bans 3rd party repairs/parts).