I also used to use your rule: if it was bootlocked then I wouldn't buy it.
However that rule has never helped me use CyanogenMod/LineageOS etc because:
Galaxy Nexus: Screen burn in (OLED blue) and obsoleted OMAP processor so didn't update, then gave to friend's kid.
Nexus 10: Kept at factory 5.1.1 for testing at work. Then tried updating this year but flash partition was too small to install gApps.
Nexus 7: Gave to family member who updated it, but it then died from slow flash hardware issue.
Nexus 5: broken frame (leading to common hardware faults) - no point updating.
Nexus 5X: Never used much - can't use as daily driver due to perceived risk of boot looping - so it is sitting in drawer at work.
Moto G1: Gave away a long time ago
Moto G3: Broken by teenager
Moto G3: Gave to friend (still in use - but can't unlock G3 any more)
Moto G4: Was a present to a friend (still in use by them, they not likely to update or want update)
Edit: Some of the devices were bought cheaply (second hand) and more than one device was paid for by work as we always need recent Android devices to test App releases on.
The Nokia phones are part of the Android One program which means the are guaranteed software updates for two years and monthly security updates for three years which is the same as the Pixel phones.
