This is a lousy idea for a smartphone, but it has potential as an industrial automation and robotics controller. Those are built up from lots of little modules, but the mechanical and electrical standards are decades old, and systems are too bulky. Think of this as a replacement for Arduino "shields", too.
Actually its the right application.
Just not in the way most people are thinking.
Modular design leads to modular construction. Modular construction leads to lower prices via economies of scale. Many ./er's aren't old enough to remember when computers were monolithic pieces of silicon like phones are today, a single assembly with everything soldered in and not replaceable. If something broke, fixing it was expensive, If you needed anything bespoke it cost an absolute fortune. Now everyone and their dog (well, except Apple) offers many options for any run of the mill laptop, ordering a custom machine from Dell is easy, every corner computer shop can offer you a bespoke desktop at competitive prices because components fit together on standardised connectors like DIMM, PCI-e, SATA and USB.
As will it be with phones, Samsung, LG, et al. will simply assemble them out of component parts that simply slot together. Designing new phones will become simpler and easier. Having to produce custom radio's will be as simple as swapping a module. This is where the average person will benefit from lower prices.
Beyond that, there will still be people who upgrade. Computer component stores have not disappeared because Acer and Toshiba sell laptops that dont need extra bits. People still upgrade their hard drives, video cards or even buy entire bespoke machines. The same it will eventually be with phones, need more storage, get a storage module. New radio technology, get the new radio module. Want a mini HDMI port... you get the idea. Not everyone will upgrade their phones... in fact the majority wont, but there will be enough people who will to justify these modules selling to the general public.
Phone repairs, goes without saying this is definitely the way to go.
Modular phone designs will happen, not overnight, maybe not even in the next few years but it will eventually happen.