I quite honestly don't think Apple's likely to be that worried. They operate off on their own OS X planet anyway and are driving Mac sales largely through introducing the brand to people through iOS devices. Quite a lot of people I know (who aren't long-term Mac users) picked up an MacBook or MacBook Pro after having had a good experience with iPhone or iPad.
This is much, much more of a threat to the Microsoft OEMs, in the same way as the Google Nexus phones are a threat to Android manufacturers.
I would be surprised if they have a major impact on Apple, as I don't really think MS is in the same league in terms of brand perception. Like them or loath them, the fruity one is an iconic consumer brand and Microsoft's brands, other than maybe Xbox, aren't and never have been in that space really.
What Apple has done is turned a rather utilitarian geeky device - the laptop, into a highly sought after, good looking consumer product.
Competition's good for the rest of us though, so, bring it on!
The major thing that will make or break this is Windows 10 though. Windows 8 was really jarring to consumers and I think it did frighten a lot of people into either not upgrading or in some cases moving over to the fruity side.
I think the biggest threat to Apple is their own arrogance. They have treated a lot of pro Mac users very badly over the years and I think that's where you could see the polish starting to wear off. Apple's connection to creative media professionals is very much part of what drives their brand kudos as it's those people who are trend setters.
Killing apps like Aperture and dumbing down FinalCut as well as abandoning people with lack of servers and all sorts of things is really what will do the Mac platform damage long term. Even if they're not super profitable, I think they need to see them as part of their marketing budget.