I do see your point in that the userbase needs to be sufficiently large to support a larger number of (redundnt) modules. I do however see three things that could be tackled with more modules:
Firstly there will always be those users whose priority is to spend little money in their phone, whereas other users rather care about performance or specific features (better camera, more memory, etc). More modules would allow to cater to all of those groups with a single, modular device (which may as a sideeffect ultimately increase the userbase, further supporting more modules).
Secondly more modules could allow for a certain level of redundancy by choice, in order to reduce the risk of something like the FP3-fingerprint-sensor-desaster happening again.
Thirdly a finer granularity of modules would allow for even more targeted repairs increasing sustainability and decreasing cost of repair.