> Pages are stored in a folder structure, like in an outliner, and can have attachments.

I wish this model worked for me, but the concept of "pages" just doesn't. I would immediately get into a situation in which I'd need a sub-page for an aside.

Is this a problem for anyone else?

I am currently using Dynalist to solve this, but very annoyed by its proptietary nature. Is there any free software that successfully implements this "infinite outline" approach?

I've actually never encountered that issue, but because Zim is a wiki style editor, you could create links to different pages for asides. It's still another page, but at least it can be all grouped together.

I have found Zim to be a great tool for note taking and creating a personal knowledge base. I can create a link to a more detailed explanation from any page if needed, or write down examples separate from theory.