But wasn't Windows Phone 7 supposed to be a radical "game changer" too?

To quote someone else on Google+ on this subject:
"Yes, WP7 is a failure. When a company the size of MS launches a device and it struggles to break a single percent of the market despite having the weight of industry leading companies like Samsung and HTC for over a year, it's a failure..."

The problem for Microsoft is PRECISELY that the game has changed... several times.. and EACH TIME they never seem to be successfully able to entice customers to their platform in the face of better supported, better managed, more productive and far more attractive competition. MS couldn't beat Palm, Symbian, iPhone or even Android in its infancy when MS was STILL faring poorer--it has even less hope today. Windows 8 is likely to be no more a game changer than Windows 7 was--especially when there are so many other far more successful game changers already out there coming down the pipe to compete with it.

Edit: You know, I forgot to address your point about how it's still too early in the mobile game to declare them a lost cause: How long has Microsoft been trying to put out a tablet? So this begs the question: How much of a head-start did they need to continue to lose the mobile computing competition to these upstarts that suddenly came along and sold HUGE numbers in short time?