+1 to plain text + encrypt and cloud sync as @amorphous said. I want these files to stay with me during my lifetime.

I do the same thing, except that I recently made a simple editor UI that shows me the entries of the same day of previous years when I'm writing today's entry.

I've only been journaling this way for a couple of years, but for me this is a good way to review how were my life and thoughts in the past. Reading previous entries gives me more chances to reflect. Many times I feel I've grown, many other times I detect I still suffer from the same issues and this prompts me to take action to correct them.

You don't need a UI that does it, but I suggest reading back some of your previous entries from time to time.