Although I like the idea a lot, I don't think it is very feasible at all. I see several issues with the convept:
Efficiency: Designing a phone architecture from the gound up allows the developer to optimize efficiency which is important for portable devices. This concept works better for PC's for instance because efficiency isn't as big of a factor there.
Size: because all the separate parts are mounted ins eparate units, withe xtra large sockets and such to allow installation, that thing will be huge. Modern phones are small because the manufacturing allows tiny parts to be packed very dense with advanced equipment.
Availability: Without a large market share, its versatility and the abundance of different modules will be very difficult to maintain.
Complexity: The base PCB or motherboard will have extra sockets which aren't used unless you can make everything use the same interface. In a computer, there are usually a bunch of extra sockets for PCI, PCIe, USB, DDr2, DDR3 etc for different things. The base board needs to be able to connect to whatever interface the part you want to attach uses, hence the sockets will be a problem and also constantly need to be upgraded, along with buses and such.
Compare this to a PC where many of these reasons often lead to people buying new rigs entirely when upgrading. It is often not all that great to just upgrade the GPU without also upgrading the CPU, adding a bit of ram, needing to replace the motherboard and PSU because of it etc.
