They could have so easily gotten around that by just calling it "Metro" and doing a clean break. Then just promise everyone that windows isn't going away and imply that there will/might still be a windows 8 which would be a continuation of windows as we all know it for PCs.
Then all the ARM devices (phones, tablets, etc) and also XBOX uses "Metro". Just Metro, no mentions of Windows. If people like it, THEN windows 8 could use parts of Metro or start to resemble it in a way. (In a manner similar to the "back to the mac" stuff in OSX Lion and Mountain Lion) If not, well they can still use windows for many years just like people stuck with XP and eventually there will be a windows 7+1 of some kind for them.
But by calling the phone OS windows phone 7 and by completely changing windows 8 people are in a panic. I can only imagine what their "enterprise" customers must be thinking.
Plus now the phone OS is "version 7" which coincides with the old pc-os style windows but it is the only thing actually using Metro and windows 8 has no mention of metro and is the only one that's new style.. so is 7 or 8 or 360 or 720 the number to associate with the new style? Should any number be associated with it? If not, then why _start_ the completely new phone OS at number 7? Why call the near complete break version 8? etc.
tl;dr: If they did a complete break and didn't call it Windows, then no one's first reaction would be "Jack of all trades, master of none."