A lot of people don't like the Office ribbon interface but prefer it myself.

Microsoft makes a lot of money from Office so it better be modern like looking. Proprietary software in general does a better job with UI design then FOSS software. The Windows 8 Modern UI on desktops or laptops is a big exception to that...

Word 2002 is still the standard where I work mainly because of the insane UI change of Microsoft.

Why would LibreOffice copy Microsoft when its main compelling advantage over Microsoft is to avoid this mess ?

I use OO/Libre Office in part just because they did not follow MS' lead on the ribbon. I have never found the ribbon intuitive personally.

I'm not at all saying that LibreOffice should copy Microsoft. I'm not fond of the ribbon, and I've never heard of anyone who likes it.

Modern design does not require following any one company's lead, but it DOES require innovation. LibreOffice's UI does not innovate.

Most people dislike the ribbon interface.

Microsoft have a patent on the ribbon bar. You are meant to ask them for a licence if you implement one. Unless you are using a Microsoft control.

Also the interface takes up screen space on the horizontal when we have wide screens. A sidebar would be a better use of screen space.

They started iterating about 7 years ago, based on user feedback and ignored people whining.

UX is the most under appreciated area of open source software. It requires change, following/setting design trends and not listening to the majority.

Not listening to the majority, on the other hand, may lead to your project being forked. Take Gnome 3 as an example. This in turn may lose you a significant amount of developers so that you have two projects that each are weaker than the original one. This seems to be an inherent problem with projects like this. The only thing resembling a solution is to have a strong project leader who is able to convince people to trust him or her on radical UX changes.