Your suggestion is precisely the same as the recommendation I made several years ago- to show notes in the tree in order to more efficiently use screen space. The current display interface used by Joplin and Evernote occupies a considerable amount of screen space and forces the use's eye to scan the first column of notebooks, then shift gaze over to the next column of notes. This seems quite inefficient when the notebooks and notes could be easily incorporated into a single tree. Many of the leading note-taking apps use this approach: Rightnote, UltraRecall, MyInfo, Mybase.

I am a big fan of Joplin, but have switched to Trilium because the Joplin non-tree interface was unsuitable for my workflow. If Joplin were to implement a proper tree design, I'm fairly confident that I'll switch back to Joplin because of its very rapid development.