I wonder if its just cheaper to make the crappy disposable devices, either because just gluing shit together is easy, or because the existing business model & supply lines based on planned obsolescence are established and optimised.

Ultimately any business model based on chasing anything but the most money is going to be at a disadvantage under capitalism. Want to prioritise good products, less waste, human dignity, not destroying the ecology? Well, you're going to make less money, so you can spend less on capturing market share and you will always have a more niche and more expensive product that will be left behind by products that focus on money above all else.