I think that 20 years ago people couldn't see a competitor coming along that could disrupt Microsoft, because they were so big and into everything.
But that's because no one foresaw the coming of the smartphone. Microsoft is still king on PCs, but the PC market has been in decline for years. Back in the early 00's no one predicted the iPhone and Android, and what a massive impact they would have on the PC market.
I would argue that Microsoft actually has two core competencies today: Office, and services (e.g. Windows/Azure). But yes, they definitely went from "king of everything" to "well enterprise still uses them" remarkably quickly.
tl;dr - Something will likely disrupt Amazon, but it's difficult for us to predict what that will be until it happens
Personally, I find AWS' interface to be super clunky. In fact, I find all of Amazon's interfaces (AWS, Amazon.com, the Amazon iOS app, to name a few) to really be poor examples of UX.
Amazon's problem is that it has a very large surface area. To competitors, that can be a feature or a bug.
And so it goes :-)
They were king of the PC, imposing a Windows (and Office-) tax on every other business. They were trying to extend that dominance to the internet: Internet Explorer, CitySearch, Expedia, MSN portal, MSN Shows, MSN Messenger, you name it. They were building interactive TV, software for cars, and a mobile phone platform. For a while they were building a bill presentation and payment service. They had an e-reader platform and tried to sign up publishers for it.
You name, if it had software in it, Microsoft was trying to dominate it. They knew they were the new IBM and that IBM had stumbled, and they were paranoid about that happening to them. And here we are.