Draw.io's local file saving is clunky as hell and I'm sure even that was an absolute pain for them to get working.

I actually implemented it recently in one of my open-source apps. It's 6 lines of Javascript: Create an <a> element, set the data in its href with:

element.setAttribute("href","data:text/plain;charset=utf-8," +encodeURIComponent(content))

and element.click() followed by document.body.removeChild(element);

This works perfectly fine for saving single files. FileSystem access API is useful when entire directory needs to be stored and perhaps without the standard SaveAs.. UI.

Those users probably didn't need their RAM anyways