The enterprise revenue is steady and stable because Windows and Office is entrenched. It will be for years to come, but if other platforms gain a strong foothold in enterprise (like iOS and Android seem to be doing now), that stable revenue can quickly shring.
Remember, this is how Microsoft overtook Novell, DEC and the likes that were the kings of business computing in the 80s/90s.
And Apple isn't just one bad iPhone away from being beaten. They have consistently released good updates to their hardware and software for the past 10 years, and have built up a huge, satisfied customer base because of that.