Full modularity never made sense. The system-level tradeoffs for battery, screen, and antennas are just too astronomical.
But one or two expansion slots... now that would've been revolutionary. You could add whatever mattered to you: medical devices, specialized cameras, etc. etc. etc. Instead we're left with shitty USB or audio jack dongles or detached Bluetooth devices. Suck.
Dongles slapped on after the fact are a poor design. They ruin the ability to hold the device one handed, the ability to pocket the phone with the module attached, or add substantial bulk to cases that must be redesigned for every variant of phone.
USB is a decent electrical interconnect and API standard... But not a good cellphone mechanical modularity standard.
Possibly due to the placement of USB on most phones - at the bottom, where a drive would stick out awkwardly.
Also, MicroUSB on a lot of devices doesn't seem like the most robust port ever - I think it would eventually wear out with more than just the nightly plug/unplug for charging.
USB by itself is a terrible answer. The connector is small and weak so you can't hang a peripheral device from it without risk of breakage. The only way this would work is if phone manufacturers agreed to add mounting rails or magnets at a standardized position near the USB port in order to provide a secure and robust mounting point. But realistically I doubt that the major manufacturers would ever cooperate on that type of standardization.
Low effort modularity would be so easy... Back when smartphones were not phablets and batteries were swappable, custom back covers for fat extended batteries were available for almost any device. That's one precedent. The other are Wi-Fi SD cards that were sold before digital cameras started to integrate Wi-Fi on their own. There is another. Custom back covers for the minority who needs extra space and few connectors in the form factor of wired microSD (because micro USB is specified with far too strong mechanical reserves for this use case) would come at near zero extra bulk for those who don't use those possibilities and the minority who does have exotic needs would still be served.
Everything that would be used by more than 5% of users should still be integrated, everything else would be a waste of resources because high numbers have such a strong effect of making electronics cheap and light.
But one or two expansion slots... now that would've been revolutionary. You could add whatever mattered to you: medical devices, specialized cameras, etc. etc. etc. Instead we're left with shitty USB or audio jack dongles or detached Bluetooth devices. Suck.
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