I don't think the monthly expenses should be much of an issue for an actually successful app, I think Instapaper proved that, it was (and still is?) a good business that did not require subscriptions.

I think Apple Notes is fine for the majority of uses, Simplenote is great otherwise, Evernote is popular... I'm not sure there's a need for another, unless you're the kind of person who cares about the particular class of UX problems that John Gruber cares about.