In my mind modular phones is a false good idea. The principle is really good, but it is a lot more difficult to do that in a phone than in a desktop PC, due to the huge constraints on it (mainly size and weight)
And you are too soon reaching the limits of the technology you have to add the new modular components.

Take any phone you bought more than 6 monthes ago. Not a single one is capable to support the 41MPixel stream of the 2012's Nokia 808 . Simply because the rest of the hardware (system bus, memory speed, CPU/GPU processing power...) is not able to cope with it.

Modularity is something really hard to achieve to be able to improve your phone with technologies newer than the phone.

It can however allow to add some components of the same generation, but who will had a 720p camera to his 3 years old phone, when then new ones are doing 4k ?

Add to that that a modular phone needs a lot of connectors that take space and add weight, and to be modular you need to split some parts which don't allow as much optimisation as having a single board design, and you'll see that it will not be that much cost effective to create that modular phone.
It would be, if there were 10 phones of each kind sold, but it is produced by tens/hundreds of thousands units which makes it cheaper to produce 2 differents phones than a modular one + peripherals modules.

For a keyboard or the other TOH projects seen here it is okay, but not for core components like camera, connectivity (HDMI, USB, ...) and memory.

My 2 cents.