I won't touch Microsoft OneNote for this reason, but for me Evernote handles the "use them in 50 years" issue (at least on OSX).

All of the notes are stored on disk as html files and images, viewable with any web browser, one directory per note. On osx, they are found in:

    ~/Library/Application Support/Evernote/accounts/Evernote/USERNAME/content

for the web version of Evernote or

    ~/Library/Containers/com.evernote.Evernote/Data/Library/Application Support/Evernote/accounts/Evernote/USERNAME/content

for the AppStore version. (AppStore apps get their own mini-filesystem.)

You should be able to back this up and use it at any time in the future without Evernote. You do have to make sure you regularly sync and backup though.

Edit: I would like to add that I like the idea of using markdown, however. Evernote docs often contain undesired baggage from the original clipped html.

I have the same requirement for my notes. Looks like Laverna stores notes on Dropbox in JSON, which is fantastic.