I think the idea of dumber phone introduces with it another notion, since the phone is seen more as a utilitarian tool(not for entertainment), then it should not have to be replaced frequently.

With the above concept the entire discussion then hinges on how updatable the phone software is? Because if the phone software is not updatable, even the minimum number of apps needed on the phone will stagnate and you'll have to buy a new phone anyhow.

The single biggest point of failure in the updatability of a phone is the devices drivers for the SOC. Linux fails in this regard because the driver ABI is not stable, so you can't run drivers that the vendor launched with the launch kernel. It is this problem of updatable drivers for SOC components that needs to be solved before any idea of a long lasting utility phone becomes practical.