Right, because we all know the most important feature of smart phones is their cost savings, which is why Apple phones sell close to their marginal cost and don't have 30% margins.
And this kind of device, while it may not be interesting for many consumers, will be very interested for industrial, enterprise, or medical users.
With bluetooth you need to consider radio interference between the sensor and the main device. In my experience that increases complexity of the sensor, and is something you largely can avoid with a module that has a physical interface to the main device.
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.
There are lots of mobile devices specifically designed for industrial and commercial use. Usually they are much more expensive, but add few additional components. It would be useful to be able to build up your own devices from standalone components. But I doubt it is really in the interest of the manufacturers who want to be able to offer a complete "solution".
zakalwe2000 mentioned a lot of kinds of costs; you only mention the monetary one. In fact, your example of the iphone supports his/her point - a non-removable battery improves reliability and makes phones slimmer & lighter while also more mechanically durable.
Not everyone values slimness to an extreme, and despite the fact that non-removable batteries improve certain design margins, many many Android phones still have removable batteries, and some even reverted from non-removable designs to removable ones.
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.
And this kind of device, while it may not be interesting for many consumers, will be very interested for industrial, enterprise, or medical users.