Another benefit to postmarketOS is that it runs (close to) mainline linux.
In the android world, vendors fork linux, put their own (often badly written) patches on top to make the device work, and then stop maintaining this fork after a few years.
postmarketOS carries as minimal patches as possible and actively works to mainline what remains. This makes the "10 year support" goal very achievable, as once a device has mainline support, it will get updates as long as the linux kernel itself is maintained.
By making everything standard and relying on the upstream kernel and linux stack as a whole, any improvements made to phones also benefit laptop and PC users, and vice versa. So, we have one big platform that can support any kind of device, sharing resources so everyone benefits.