> If mobile devices would routinely last twenty years, which they very well could, that would reduce a lot of e-waste.

Unfortunately, battery technology doesn't - and even if we had long lasting batteries, we'd also need fall-resistant screens. And no matter what, even if you have a device held together by screws and allowing easy repair instead of messing around with glue and click-tabs... screens still are really expensive, making it often enough more worthwhile to take the opportunity and upgrade the whole device rather than to repair the screen.