Bit old news, but new phones are added on a regular basis. Such as the Fairphone 4, which also gives you at least 5 years access to spare parts, which is great since it’s a modular phone.
About postmarketOS:
You will still have the “vendor implementation” [in Android], which is different for every device. It will contain at least the kernel and drivers (kernel/userspace), for which you, as a user, depend on the manufacturer to keep it updated, and which will probably not be mainlined. This means, that after two years or so, when the support runs out, you will still have a device, that does not get updates anymore. Only for a smaller component of the operating system.
postmarketOS does this differently, they develop the OS as a regular Linux distribution. So no vendor dependency.