As an electrical engineer, I'll just go ahead and say this would be an absolute nightmare of a product to design and I really don't think it would be as convenient as it's making itself out to be. The gentleman who came up with this graduated from a design academy, not an engineering school, so I doubt he's put any thought towards all of the insane engineering hurdles present in a project like this. Also, the phonebloks website has typos and crappy grammar, which is unprofessional and bothers me personally.
Small and big companies develop and sell there bloks.
Modern smartphones are probably the most high-tech consumer products out there. For just one phone design, engineers bust their asses to get everything working together correctly. Electronics isn't just about connecting wires together willy-nilly. For instance, a big part of electrical engineering is planning out acceptable trace lengths and paths, because, in a signal-dense environment, there are going to be a ****-ton (technical term) of non-ideal effects. That means that the actual physical layout of your components is not something that can be safely altered.
And that's literally just the first thing that came to mind. I can think of way more problems that would be associated with this kind of system. This is a bad idea and will never see the light of a testbench.