is it really that important for a non-emacs client to support 100% of org's features?

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.