And this kind of device, while it may not be interesting for many consumers, will be very interested for industrial, enterprise, or medical users.
To put it another way, if you were running a medical devices company with limited resources, would you bet on this or bluetooth?
I'm hoping that project Ara will become the perfect test bed for sensor module development. Useful after the initial validation stage of experimental stuff on a breadboard but before the stage where you have established enough customers that you can afford to develop a completely integrated and standalone IOT device.
Then they'll be leaving room for a newcomer to do it.
There is a crossover point wherein overhead reduction is "good enough" and when other factors about the phones begin to dominate.
The iPhone doesn't sell because of a non-removable battery, and the huge number of users who use bulky cases to protect fragile phones, and add-on battery cases, which increase weight and spoil the industrial design, tend to support the case that some people are willing to trade off thinness, weight, or cost for other capabilities.
I don't even think this is controversial. If you look at the number of users asking for microSD whenever a device removes it, or the people wanting additional clip-on lenses, there's a hunger to extend the phone platform.
Now, whether that happens over an integrated UniPro bus, an external lighting cable + case, or bluetooth, is an implementation detail of convenience, ergonomics, and bulk. I personally would rather have upgradeable camera modules or additional battery slots than "Morphie" cases and weird protruding clip ons that weren't designed to work with the builtin optics.