Without Scott’s unusual intellectual skills and ferocious focus, iOS wouldn’t have been born as an OS X derivative
Apple Culture After Ten Years of iPhone
Jean-Louis Gassée

It’s been popular to dismiss Forstall’s contributions since his firing on the usual “Apple can do no wrong” premise, but it seems pretty clear iOS wouldn’t be what it is today without him. The vision and execution happened under him, and those early versions of iOS were rock solid. Granted, it’s more complex now, but there are types of bugs we never saw up through iOS 5 or 6.