The problem with a modular phone (no matter who makes it) is there are big design tradeoffs. To increase the modularity, especially from an end-user perspective, you have to give up some combination of size, cost, power, battery life, water-resistance or other features.

It just probably isn't worth it, especially for the case of a partially damaged device where there are probably better solutions... Like a "loaner" program or 2-hr on-site repair for most repairs. These would increase the cost of repair (or repair insurance like Apple Care)... that's a tradeoff in its own right, but at least it doesn't impact your day-to-day usage of the device.