too much work to keep menus?
IIRC, when Microsoft was asked why the ribbon was obligatory--why they couldn't make it possible to use either the ribbon or the old menu--the reply was that it was too much work to maintain both. That always seemed bogus to me.
Now for LibreOffice we read: "there are now a bewildering number of options for menus and toolbars. You can select Standard Toolbar, Single Toolbar, Sidebar, Tabbed, Tabbed Compact, Groupedbar Compact, and Contextual Single. You can also combine various options, such as choosing Tabbed UI, which is reminiscent of the Microsoft Office ribbon toolbar, and then adding the traditional dropdown menubar on top." So what Microsoft couldn't do because it was too much work, the LibreOffice developers are doing, and then some.
(I do suspect the Microsoft reason was really an excuse.)