Whole heatedly disagree. A thinner device doesn't automatically mean material costs reduction. Sometimes, miniaturization can cost more. A smaller or thinner device becomes more error prone at the manufacturing end, meaning a higher defective rate, even with assembly line automation. You should read into supply chain management for more information on that.
Apple's (or more realistically, Tim Cook's) pursuit of margins is what can be attributed to reduction of material costs, and the recycling of old hardware into "new" devices (iPhone SE). Not saying that's wrong - any company has a right to increase margins - but that's more realistic.
While I agree with you that miniaturization has its own set of costs
(R&D, production investment, etc.), I still stand by my initial reasoning.
Yes, pursuit of thinness isn't free. However, it's one that has its own set of gains as well.
If a device can be made thinner and maintain the same level of performance
(computation, endurance, etc.), then why not? Most people won't complain.
At the same time, there are gains in user comfort (lighter weight),
bigger profit margins (materials cost reduction), etc. (Bragging right?)
Curved-edge screens, however, don't seem to have any sort of realistic gains.
But it comes with its own set of manufacturing headaches.
(I work for a display manufacturer. Cannot say which one, but let this suffice for now..).
**EDIT**
That said.... Apple very well may introduce curved edged screens in the future.
(I have no insider knowledge; this is my personal opinion).
And they may have their own justification behind the decision.
Apple has done things in that past that's left its users perplexed...
All we can do is wait and see.