Client applications would have no consistency, and cross-application integration would most probably be non existant
Programmers would have a tough job with their developer tools now. Without Windows' UI and API, the same level of User Interface richness would still need to be acheived
The hardware ecosystem would be felt the hardest, when there is no Windows OS. There will be no dominant OS, and vendors will seek to align themselves with the perceived market leaders of the day.
What will happen to technology when there is no more Microsoft?