You say this as if there isn't a widespread base of consumers who don't want to tinker, don't want to upgrade and don't want to assemble and the only problem is manufacturers don't want to make modular phones.

Frankly a modular phone sounds godawful. I work all week and build stuff in my spare time, I don't want yet another thing I need to maintain and that's why I have an iPhone.

There's no reason we can't have proper disposal methods and environmentally sound manufacturing -AND- have single-piece prebuilt devices. I'd argue that making stuff modular wouldn't even solve that problem in the first place, you'd just have piles and piles of PIECES of phones instead of piles and piles of phones.