If my smartphone were modular in the same way as a desktop PC, I would worry that minor bumps would cause disconnections or shorts. In theory I'd love to upgrade parts at low cost, e.g. upgrading to an AMOLED screen, but ease of disassembly doesn't seem compatible with the durability/weight/size requirement of a device I carry around all the time.

In practice, it's easy to buy a new phone every 1-2 years, and give the old one to a relative.