Metro would be ok as a concept at least if it was a Windows component you could choose to install.
Look at Windows Media Centre for example - outside of a media PC there will be many Vista and 7 owners who never use it, and aren't affected by it even being installed. There are others, such as myself, who use WMC daily in the lounge, on a PC that is sat inside a AV cabinet operated by a remote control.
This is critical to understanding why Metro is such a failure. People with desktop computers will likely be sitting some distance from their monitor, and it would be uncomfortable in most cases for them to operate its touchscreen when it sits vertical on the desk. Notwithstanding that usability issue I would assume that it is still the case today that the vast majority of Vista/7 users do not have touchscreens, and in my experience Metro is pretty underwhelming without one. The use of a touchscreen is antithetical to using a desktop computer for the most part, yet MS seemed to think that the transition would be fluid and that the marketplace was just crying out for someone to fill this void.
This would all be just a misstep if it were possible to get to the main Windows desktop and stay there and retain all of the functionality you had in Windows 7 (Start button, etc). Instead Metro apps and utilities drop you to the old desktop seemingly on a whim and without warning, which is quite jarring, and you can't even really choose to stay there if you wanted to with ease (at least not without third party utilities to help you recreate the old UX). It is quite a shock to drop from Metro to the old desktop, the UX is completely different - which is fine for a seasoned user but is it really the experience MS wanted people to have?
That W8 drops you to desktop with a totally different UI smacks of MS really not having a clear direction or dedication to Metro, which is something you can't really say of Apple for example. Apple are notorious for having a walled garden approach to their software, and the OSX UX is very much "they'll take what we give them", but Apples customer base is used to that UX, they are familiar with it, and it is not change for changes sake.
Metro would've imo made a great Windows component in the same vein as Windows Media Centre - something you can choose to install or even boot to IF you want to, as it is it's an affront.