We never got to a point where you would have total interoperability between ODF, DOC, DOCX, RTF, etc. I think this one is even harder. Everyone want to show of as the best/fanciest/etc. HTML + CSS + SVG + JS sounds like a combo that could do it all, but then you realize that not even all browsers render the things in the same way.

So I would say that you are right - vendor lock in.

I didn't think of that. It makes me think that my question is just a more specific case of: "Why do some applications converge on standard file formats?"

I.e. Why are PNG, JPEG, HTML and SVG ubiquitous, but not ODF?

You gave some reasons for why document editing didn't converge on ODF, DOCX etc., but I wonder if there are some common factors that could let you look at a potential application standardization opportunity and decide if there's a chance it will happen or it's best to let it be...

Partly it’s commoditization (https://en.m.wikipedia.org/wiki/Commoditization). There’s no money in differentiation of image formats (in part because browsers dictate what formats people can view, but also because the current set is good enough for typical use cases). Note taking apps seem to think there is in note taking.

Partly it’s not true. Tools such as Photoshop have their own image format. You could even claim PDF is the format we converged on for exchanging rich text data.

ODF and DOCX are at "war". DOCX claims to be open, but it's not everyone cup of tea. I personally like it.

I believe Xournal++ will be what you are looking for (once it is out of beta).

While not open source, there's also Styluslab's Write which saves the files as svg so you can open them anywhere.

Another vote here for Stylus Labs Write, you can make your own SVG templates and re-usable elements. The only limitation with it is that you can't enter text.

Wow, Xournal++ looks exciting!

At least I haven't found it myself. I find OneNote overall worse than its "competition", but since it's one of the very few cross-platform handwriting apps, I'm still using it.

It did make a switch from iPad to Samsung Tab a breeze.

Check out the TextBundle file format.

http://textbundle.org/

“The TextBundle file format aims to provide a more seamless user experience when exchanging plain text files, like Markdown or Fountain, between sandboxed applications.”

A bunch of apps support it. There’s a list of the front page of the website.