If you make a bootloader unlock require a full wipe/rekey of the device, and make unlock status visible at boot, most of the "someone might unlock my bootloader maliciously" concerns go away.
Fair point, but that solution doesn't address the market for theft, so there's a tradeoff there.
Fairphone actually does this. My FP3+ displays a red bar with an open padlock as long as the bootloader is unlocked, and when one changes the bootloader lock one way or the other, the phone wipes itself.