It doesn't need the same interface
All you need is the same experience.
See: this is why I think Windows 8 could have worked IF Microsoft would actually have used their brains when setting us up with it. They should never have replaced the start menu with the start screen, but instead made the start screen ("Metro") into a new 'dimension'.
So basically: a layer "on top" of your desktop where you could switch out into. Comparable to switching desktops. Then they could have made that into something fully (or closely) comparable to the interface of the Windows Phone (a device I still genuinely enjoy). So your Windows desktop would merely be an option within the Metro interface. BUT... when working with said desktop there would be no need to switch to this extra layer.
But as soon as you got a phone it would make sense because this would allow you to alter your setup with that of your phone. I dunno: maybe even allowing you to swap (drag and drop) data between your Windows desktop and your Metro environment.
That could have worked. 2 separate environments each aimed at what is most needed but combined within Windows. But this... Meh.
"With Fluent Design, Microsoft has a new goal. It wants to discard the multiple, disparate user interfaces of the past for a single, focused, and attractive user interface that will woo users used to cleaner and focused interfaces from macOS and mobile platforms."
No, it wants to design something new to appeal to users in order to sell even more stuff to us. We already had Metro, why would we need something new yet again?
Does Microsoft even realize how much generations of phones people bought? I now refer to those who are more into WIndows Mobile than I am (read: who bought into every generation)?
Who cares about the past: let's do something new yet again! Ready your wallets everyone!