A natural progression? Possibly.
Then again, half the initiatives, product directions and announcements in the industry are responses to Apple, or anticipation of what Apple might do in the future.
This is how it's been for years. These "natural progressions" mysteriously seem to come after Apple has done the natural progressing first. Then, after witnessing Apple's results, the industry deems it "safe" to move ahead en masse. Suddenly everyone releases an iPad competitor. Suddenly everyone is out with an "ultrabook", etc. And they usually don't get it right the way Apple did it the first time around.
Apple sneezes, everyone grabs a Kleenex.
That's the benefit of doing almost everything in-house. There were products similar to MBA and iPad before they were launched but they were never popular. Why? Take a look at e.g. iPad. What other company has the skills for hardware and software? Samsung, LG, HTC, Lenovo etc are just hardware companies. Nokia comes to my mind but they would still be selling us dumbphones if it was only up to them. Microsoft is software only and Google wasn't into OSs until a few years ago.
I think Apple was the only viable company that could come up with a totally new product category like tablets. They have the people for hardware and software. We had "tablets" running Windows before iPad but due to the lack of optimization, they never saw the mainstream spotlight.
As for MBA, the first revisions weren't very popular. It ran hot and the SSD version needed you to take a mortgage. The 2010 update made it truly popular. SSD prices had gotten low enough and Intel&NVidia had components that were finally cool enough for such a small machine. Other OEMs didn't jump in in 2008 because the market was much smaller. Now others are following Apple and coming up with Ultrabooks. This doesn't really have much to do with vertical integration but Apple was simply brave enough to make such device in 2008 and wait two years for the product to become what it should have been in 2008. Apple already had an ultraportable in 2008, that's why they were the first to make them popular. Others saw the success of 2010 MBA and are now following the path of that success.