Microsoft doesn't have the aesthetics to build something like the iPad; it's completely foreign to the way they approach almost all of their products. They're not known for the thing that makes the iPad successful: simplicity. Simplicity in hardware, software, and product positioning.
If Microsoft made the iPad and its ecosystem, it would be successful. The trouble is, they didn't -- and they don't seem to have any cohesive, likely-to-succeed strategy to attack it.
Part of the reason for the quick uptake was what already existed that made it immediately useful.
This is a single feature that very few companies include in their products.
Usability is not a trade secret, it is simply not in the culture or preferences of the leadership of microsoft or google to make products with usability. Sony used to, but is more hit or miss these days.
Asking, "what if the iPad had come from Microsoft first" is a question that doesn't work. Microsoft introduced their iPad almost a decade ago, with windows for tablets. It was q flop.
If that doesn't fount because it isn't like an iPad, then the question is like asking "what if an orange tree bore apples?"