Fair point given how things usually go in the private sector. In the public sector, especially DoD, the rules of the game are changed.

What happens if or when AWS or Amazon ceases to exist?

Well guess what, they can't. It's now a national security issue so the necessary services will continue to be provided by a KTLO crew or DoD would facilitate a transaction where another company could absorb the needed technology and be contracted to support it. DoD is the biggest company on earth. FY 2019 budget is $686B.

DoD has a pattern of keeping mission critical end-of-life systems running and supported.

I'm currently advising the US Navy on moving some COBOL systems to Java. Those systems have been lovingly supported by lifelong Navy civilian employees for 40-50 years. Those people are now in retirement age and so they're "replatforming" to Java.

There really is no corollary in the private sector. The default for government systems has been the people who built it stick around and maintain it until they retire. For DoD the biggest risk isn't old technologies and changes in the private sector, it's brain drain when people retire.

You might ask yourself, what 20-something wants to work for the US Navy, as a civilian, and work on a Java implementation (ported from COBOL) for the rest of their career? Well, I'd like to introduce you to them. These guys could work anywhere but they like the Navy. They like that their work matters, it's important, and it's stable. The pay is below market but it isn't terrible. Keep in mind DoD isn't all in DC, many offices are in inexpensive places to live.

Frankly I'd love to work on systems with the meaningful impact that theirs have (non-combat in this case). Read about 18F and what it was like for the people who joined and how they stuck with it anyway.

This underlines how so much of the work and contracts at the DoD don't really revolve around defending the US than being another way to hand out tax payer dollars to already well off corporations.