Why have these features built in if you can have a usb back case/device with them? There are many back cases for specialised functionality like PoS, Ir camera

Why have these features built in if you can have a usb back case/device with them? There are many back cases for specialised functionality like PoS, Ir camera etc. Also, with many cases you can have devices connecting wirelessly via bluetooth - e.g. I bought a bluetooth trichiscope recently. Making a specialised phone instead of a plugin is a way more expensive option. And in risky if the market is so small that nobody did a plugin first. There were I think two companies that tried to build a modular phone where you could eg replace camera with a module of your own design - the issue was that they were bulkier and more expensive than a regular design - you can’t cheat physics. Also, how much more would you be willing to pay for such a phone? Could you pay 2-3x the price and be happy with upgrades every 3-5 years, and the phone having electronics that are 1-3 years behind the top of the line ones on the start? Because that’s the reality of production with niche products.