This is one of those ideas that look good on paper (or youtube) but just isn't practical. There's absolutely no spare space in a modern phone. If you make it modular, each 'module' has to have enough space for its component, PLUS space to bring it up to the next module size. This will make the phone HUGE. The concept of an upgradable device is, however, great. Just not in the manufacturer's interest. Apple lose out i…
My Treo 700p was >3x the thickness of an iPhone 5S, roughly the same width, 7mm taller (not counting the antenna, which was, by far, the most annoying physical part of the phone), and weighed an extra 68 grams. My manager around the same time used an HTC Windows Phone of not-dissimilar size and weight.
Phones are smaller because they can be, not because they have to be.