> Instead most users are reliant on the device manufacturing seeing the financial incentive to provide updates.
Most users are reliant on this regardless. Few people possess the technical ability, much less time, to perform these tasks. Making the platform "open" and pointing to that as a solution would also be a way of weaseling out of that responsibility to users.
The number of people who could use a fairly simple third party to to update their operating system on their phone is much, much larger than the number who can figure out how to obtain root access or unlock their bootloader using a technique that varies depending on exact model number and firmware version.
Obviously there are users who will be left behind, but that is an issue for novice users of ANY free and open source software.
Most users are reliant on this regardless. Few people possess the technical ability, much less time, to perform these tasks. Making the platform "open" and pointing to that as a solution would also be a way of weaseling out of that responsibility to users.