The horse and buggy worked so well in the past, why introduce automobiles?

(Note: Example with exaggerated contrast is used here for illustration purposes only, not for comparing size of change.)

Microsoft wants to leverage their desktop dominance to introduce people to their mobile offerings. The change from start menu to start screen fits right into that strategy. However you can get back the start menu by installing classic shell, pokki etc.

For time and the world do not stand still. Change is the law of life.

And those who look only to the past or present are certain to miss the future.

Mostly importantly, tablets and hybrid tablet/laptops are taking off and people want to use the same OS for both. And Microsoft has tried using the Taskbar and Start Menu for tablets before, and it sucked.