OneNote binary blob format, while nominally open, is supported on a limited number of platforms by a single company.
And the "limited number of platforms" includes everything 98% of people are using.
I do sympathize with your views, but I value convenience more, in this case.
I use dropbox to sync a text file from my phone and a separate file from my tablet. These text files are managed by gthnk (http://gthnk.com), which combines all of these files overnight. By the next morning, all of my notes are together no matter where I originally wrote the note.
I'd imagine it wouldn't be too far fetched to utilize a sync service like your current solution for flat files in a more real-time manner...
Also, you're right; there's no deep architectural reason why you couldn't sync more frequently than once per day.
Surely you're aware of the myriad of applications out there that will synchronize these things without issue.
Anyway, it's just one set of tradeoffs over another. In this case volatility and buttons for finicking and platform independence.
Of course, we are making it more difficult for other products to gain traction. Say, for instance, a start-up wants to offer a solution with extreme privacy without the drawbacks of an Microsoft or Apple.
It's actually one of the few Google applications (as opposed to data services, like Search and Maps) I use, since I've never found a mobile client like it, with just the features I need and nothing else.
After getting used to the general structure (Books, Pages, sections), and ignoring most tools, it grew on me.
Which, like the OneNote version in Office on OS X, can only sync to a Microsoft account through OneDrive and can not even create local notebooks.
That may be a show stopper.
Now on OS X it will go into offline mode and save to an ever growing local temporary file, but you can't choose to create a local notebook.
I had to use the full version of 2016 that I could get with Office 365 on my Surface to save to a local directory.
https://support.office.com/en-us/article/Frequently-Asked-Qu...
https://answers.microsoft.com/en-us/insider/forum/insider_wi...
http://arstechnica.com/apple/2014/03/new-free-onenote-for-os...
A good example of what could be a subtle change making a big difference is both creating new note & deleting an existing note happens instantly now, past versions there was some a brief lag. This is part of the reason why even a little bloat can really suck in anything you use multiple times a day.
Since I also use Windows the problem I have is getting to those notes on my Windows 10 boxes. The only fast way I've found is to log into iCloud.com. Not perfect but good enough.
A) open-source it
B) allow me to host the data on my own server and syncing is automatic
C) Make it trivial to open/edit it in a terminal (inbefore "but you won't be able to do pictures!")
I won't be using it myself.
And obviously OneNote itself isn't open source, but the file format is, so it's not like your notes are locked into the Microsoft ecosystem.