There are already (to me) very useful places where I can get OneNote where I can't get emacs -- Android and iOS. I don't expect to ever see Emacs on iOS, and I tried it on android but it doesn't work well at all.

Not sure why you think emacs org-mode will be around forever. While I'm sure emacs will be around forever, I've had vim (not emacs, I don't use it I will admit) extensions which rotted as new versions of vim came out.

Not Emacs but Org-mode.

There is http://www.orgzly.com/ for Android. not sure about iOS.

Not sure how up-to-date these applications are, but there are Android and iOS applications designed to read and edit org-mode files.

http://orgmode.org/manual/MobileOrg.html

Honestly, I'm working on reducing my dependence on computers/pen/pencil/whatever for todo lists and organization. I try and keep it all in my head -- if I can't, that usually means I'm biting off way more than I can chew.

I am all for whatever works for you, but GTD and some other psychology texts make an argument against this: that writing things down allows you to release them from your memory and thus focus more clearly, which GTD refers to as a mind like water.