Emacs with Org-mode. Emacs has already been around for longer than that, and will surely be in 20 years. Org-mode is a part of Emacs and isn't going anywhere.

Emacs with Org-mode. Emacs has already been around for longer than that, and will surely be in 20 years. Org-mode is a part of Emacs and isn't going anywhere. Plain text with extensive markup support, including inline images, code, inline HTML/LaTeX, etc. Easily export to HTML, PDF, blog, etc. Easy to store, backup, sync, version-control, etc.