So in the early days of a new market/category of product, the products are highly integrated, for several reasons. One is that the market is still new, so there isn't much of an "ecosystem" to begin with. Another is that the company with the "first mover's advantage" wants to keep stuff proprietary as much as possible, and another is that the product still kind of "sucks" in some areas (camera, battery life, in early days of the iPhone for example - compared to the traditional competition). So they need to make everything work as tightly as possible, to squeeze all the possible optimization out of it.
But eventually, the market becomes mature, the ecosystem grows, and the products become "good enough" for most people. So much of that extreme optimization or need to keep everything proprietary and in-house isn't needed anymore, and you actually start getting some advantages from the modularization of the market, such as buying a better modem than you can make from a "modem company", and so on.
For a while I wasn't sure this was going to happen to the smartphone market (ignoring the fact that there has been an increasing trend towards customization through colors and whanot), because for one the smartphone is a very tighly put together product, and it's hard to imagine how it could've been separated into a dozen different pieces without being junk, and two, for a while the trend was towards increasing "closeness" of devices, rather than openness.
But it seems it's going to happen, and ARA looks just about right (I wasn't a big fan of the Phonebloks pin-model). Still, even if the strategy here is "correct", and most likely on the right side of history, Google will still need to excel at execution, and make sure using such a phone gives very few disadvantages compared to a regular phone, but many more advantages (being able to use whatever camera you want, without buying a $700 phone every year, and so on). Otherwise, people could be turned off by the initial version, and then it will be a lot harder to convince them what a good idea this is. But for now I'm optimistic.
[1] - http://www.amazon.com/Innovators-Solution-Creating-Sustainin...
Then you're probably addressing the wrong market. I found _The Innovator's Solution_[1] by Clayton Christensen pretty useful to understand that. You want to address a market that is underserved or not served at all by the existing big players. Often, it's by providing a service that is much simpler, much less powerful, but cheaper and/or more accessible than the existing solutions.
[1]: https://www.amazon.com/Innovators-Solution-Creating-Sustaini...