I've seen this kind of thing appear a few times a year and, while they are cool and I like Markdown, I find I stop using them rather quickly in favor of things like the macOS Notes app or Slack because of things like convenience, ease of sharing, better search, more ease of mind that my notes won't one day disappear because of a side effect of clearing history or because the developer didn't understand browser storage API quirks. For example, I used Dillinger.io until the interface was updated and my notes were gone. I seem to remember another one that interacted with gist.github.com until one day it didn't. Hurray.
I think a company around a Markdown based app or extension would be good because other solutions don't embrace Markdown, and the ones that do might be a flash in the pan.