I would note a few things...
First, dash is meant for streaming, not creating local standalone files that some website rips from utube. Those sites should not output the files as dash files. It's at best inappropriate and imo, lazy on their part.
Second idk if windows media foundation:
https://en.m.wikipedia.org/wiki/Media_Foundation
Supports dash or not, I know tho that winamp often relies on it.
Finally I agree that winamp should support dash, esp since it can be streamed, and other apps do either way, but I fear winamp is dead in the water for now.