I've owned several Nokia phones in the early 2000s. It was dead simple to replace a battery. My 70 year old aunt who is a tailor by trade knew how to pop out the back, and insert a new battery. There's absolutely no reason a screen replacement can be made as simple. The Fairphone series has already achieved this for screen-replacements (albeit with a bit more difficulty than a simple battery replacement.)

Even if the majority of people are unable to do these part-replacements themselves, it is still a massive improvement to make them easy to perform. The reduction in expertise required to perform these replacements would significantly reduce the cost of these operations while simultaneously reducing e-waste.