There's a patch reducing write amplifications (when caused by indexes), by a significant degree. Unfortunately it didn't quite get ready in time for the feature freeze of 10 - as it affects the on-disk format, we considered the risk to be too high.
As the author of the patch I don't quite agree to it. But it's true that the patch did not receive adequate review even though most of the on-disk changes were known and coded at least 7 months before the feature freeze. So it's hard to tell which part of the patch wasn't ready. But there is always next cycle. So lets work towards getting it ready for v11.
