First, bringing up the computer store is a red herring. The store where you buy your phone has nothing to do with it.
The store where you bought your computer is analogous to the carrier. You have to wait for both your manufacturer and your carrier to offer an update to your phone. This is true with even some Nexus devices. You had to wait for Verizon to update their version of the Nexus.
Second, if you bought a phone from a carrier that doesn't unlock your devices, that's your fuckup, similar to if you bought a device that the manufacturer would not unlock. Don't be a fuckup.
Your phone being unlocked by the carrier still doesn't mean you don't have to wait on the carrier and the manufacturer to update your device.
Third, Apple makes it effectively impossible to support their abandoned devices, at least with some Android devices it is possible.
It's possible to jailbreak every iPhone that has been abandoned by Apple. The latest iPhone that is not supported is the iPhone 4 released in 2010.
They do if they are running Windows XP and are unwilling to pay for a newer version of the OS. Android users may or may not get an update, but their updates are free.
So it's better to not have the option to update your OS than to have the option to pay for it? If you bought a computer with Windows 7 or Windows 8, Windows 10 and is a free upgrade.
In other words, every Windows conputer sold since before the introduction of Android is eligible for a free upgrade.