Making the screen completely modular would be one way to achieve this, since you can just replace the screen with a new one if the old one breaks in a drop. But it can be achieved much more easily and cheaply by simply slightly recessing the screen (or by slightly raising everything around the screen) so the glass never has to make direct contact with concrete in the event of a drop.
This is how bumper cases work, and my $3 bumper case has saved my $400 phone enough times that I'd never use a new phone without one. That said, I really wish this kind of resilience was built into the phone itself, so I'd never have to have to cover up the nice design and great feeling materials on my phone with a cheap, bulky, hideous case.
iPhone 4/4S did this just fine, undo two screws and the glass comes right off ready to be replaced for like $10.
iPhone 5> they started fusing the glass to the sensor IIRC so this became far more expensive/cumbersome
I've had good luck in choosing phones with cases just elastic enough to break a bit while protecting the screen (even without any kind of cover). But I never brought into the hard-case movement by Apple.
I think those are quite different: a phone that screws/clips together where you can replace a damaged screen is a very different prospect from one made of pluggable blocks.
That's not what sales numbers say. People repeatedly buy the thinner/lighter models (which also seem more "advanced to then") over heavier alternatives.
Focus most of your efforts putting the good stuff into the thin phones and then run the numbers how everyone is buying thin phones!
It _would_ add a little size/mass overhead, but it wouldn't have to add a lot. It would just cut in to bottom lines to build and maintain a consistent form factor. Allowing people to replace parts means they'll be buying fewer new units. My last two laptop purchases were definitely due to a hypothetically fixable problem in an unrepairable form factor.