Locked boot loaders
And this is one of the main reasons why locked boot loaders should be illegal IMHO.
At the very least, if locked boot loaders are allowed, manufacturers should be required by law to unlock any phones that they're not providing security updates for. If a phone goes more than a certain period - say 2 months without an up to date security patch, then it should be mandatory that the phone boot loader be unlocked so that it's then possible for others to take on the job that they obviously no longer want.
Locked boot loaders are one of the reasons why I haven't upgraded from my nearly 4 year old (but very capable) phone that is now getting weekly updates for LineageOS.
Biting the hand that feeds IT
