History has also shown us that most new ideas fail. Even good ideas.
I agree that the idea of accessories per se, attractive as it is to me, isn't enough to make a product a success these days. However I should point out that back in the day of PDAs it was normal for mobile devices to have a CF or SD slot that could also be used to add features. This was in the day when mobile devices didn't have cell data connections, GPS or even wi-fi, and it was quite common for people to add memory cards, wi-fi, bluetooth, and GPS. I have a box full of accessory cards in my attic.
Handspring, a company that made Palm Pilot clones, initially did very well with their Springboard modules which allowed you to add any kind of functionality to the base system, just like what we're talking about here. Then a few years after introducing the Springboard module Handspring stopped making PDAs altogether in favor of what was then called a "converged device" -- aka a smartphone -- without the slot. It's all about timing; Handspring was perhaps a little ahead of the curve on convergence, but a lot of manufacturers were getting pushed that way because of falling hardware retail prices made it attractive to put more stuff in the base device to keep the price high.
The standard inclusion of GPS + Cloud + Camera + Bluetooth built-in means that there really isn't a need to physically connect a device to a mobile device. The only exception is battery; there is a real need for a more elegant and secure way to extend the operation of a smartphone than plugging it into a powerbank via USB.
But I may be wrong. Maybe there's a compelling use case for a modular architecture that I just haven't thought of yet. That's why I like to see vendors trying something different, although I usually expect them to fail. I've watched tech long enough to realize that success isn't just about an idea being right, it has to come at the right time.