The problem with people who make this argument is they attribute causation to the fact that Apple was integrated when in fact Apple lost out to the PC because it stagnated. Anyone who looks at Apple's history realizes that they managed to stay ahead of Microsoft for almost a decade. If you look at the original Mac versus Windows which really didn't overtake the Mac until Windows 3.11 in 1992. So Apple didn't lose because it was an integrated provider Apple lost because they basically didn't move forward for an entire decade while Microsoft continued to move forward at a rapid clip. There are other reasons like Steve Jobs leaving and them choosing the PowerPC but in the end you can't say that Apple lost strictly because of its exclusive nature.
I'm not entirely convinced that Apple would have done better had Steve Jobs stayed at the company. It's entirely possible that he would have run the company into the ground while avoiding the other poor choices that Apple made in his absence. It's also entirely possible that during his time away from the company he learned things that allowed him to come back to the company with a fresh outlook. The idea the Steve Jobs is the sole reason that Apple can be successful, leading to the conclusion that Apple would have been successful for their entire history, had Steve Jobs never left, is wild speculation at best.
But "Windows" wasn't the product Apple was competing with, it was MS-DOS and the PC platform in general. And Apple was losing there too. Windows 3 was a big product event inside the PC world. It had little to no effect on the Mac vs. PC marketshare numbers (though it did force a lot of churn in the PC deployed base as it finally managed to obsolete all the XT clones).
