Both, which I think is part of the problem. They’re splitting engineering effort between Office 2003 style, Office 2007 (ish) style, and Office Online style UIs. They probably have less engineering resources than Microsoft Office and it shows.
I, on the other hand, really like that i can make LibreOffice Writer look like a slightly overgrown WordPad and i wouldn't like this customizability to be removed. I'm sure someone else likes to put their toolbars on the side or whatever.
And TBH the different UIs aren't really that big of a deal, they're just different arrangements of action buttons and menus, that is only a tiny fraction of the engineering effort that you'd need even for the word processor part and it isn't like they change their UI all the time.
