Between stuff like this and Workflowy, I think there'd be a market for a cloud-based version of org-mode with nice, non-Emacsy front ends that supported the key org-mode capabilities.
And some stuff, like babel (which is probably important to devs), depends on other emacs packages...
You can still always open complicated org files in emacs if you really need to, but that shouldn't stop people from attempting to develop a lighter client alternative that's more convenient for the browser or mobile, for example.
Even just using a subset of orgs features, it can still be significantly more powerful than just plain old markdown. Most of my org notes would be ok to edit with nothing more complicated than the labels+tags+checkboxes+tables features. Would be mildly annoying to manually update a few things here and there, but it'd still be far better than nothing.
No. Even babel's not really necessary.
But even implementing a broad subset might be hard. However, there is already rudimentary support for the org format outside org, so it may not be that bad.