I agree with you that bookmarks view is better. I'd even go for both. However, it doesn't work well for others. Setting it as the initial view has problems I am afraid of if sharing with others.
- Some of their viewers probably simply are too dumb to honor it. (the program not the user)
- If they have internal settings like "restore last used view", which was the default in my pdf viewer, it will override your initial settings.
- I doubt I know about all the potential problems. There must be more.
So, you just can't count on it working for other people. And, it doesn't work inside Joplin. I just don't export to pdf very often for myself and don't have faith in initial view for others.
Imagine someone complaining that there is no table of contents. You say there is. Now, given their pdf viewer, chrome, vivaldi?, Firefox, edge, acrobat, foxit, pdfxchange, etc. you need to explain where their restore last view preference is set so they can see the bookmarks.
Also, they might not even know about bookmarks or how to get a bookmarks pane.
Perhaps the people you deal with are better educated in PDF than those I deal with.