This doesn't make any sense. It assumes there's no competition between manufacturers, and that users don't switch. If older phones were actually "better", manufacturers would just make those and steal customers from anyone making the newer devices.

The phones of today aren't "built with shorter lifespans in mind", but they are built with more tightly packed components, more energy-dense batteries (to support their power-hungry CPUs and large amount of RAM), etc. Because that's what you need to do so you're not "too underpowered to run modern software." Which is necessary to sell phones to people.

If you made a phone and said "hey, this will live as long as a Nexus S, but it's not going to run FB or Clash of Clans or VR or take very good pictures (HDR is compute intensive)".... good luck selling that.