First arguing based on existing infrastructure ignores the fact your changing the system therefore any new system is a viable option. All the existing system changes is how much things cost. Anyway, for independent distributed systems you can use probability rather than fixed numbers.
That said, your losing the forest for the trees, the accuracy isn’t that important. You can still bill for actual usage. A 15 minute granularity is vastly better than a 30 day one. As long as you can kill processes you don’t need to check in the middle of every action. Things being asynchronous is just the cost of business at scale.
I'm hardly saying it's impossible; I'm saying that it's not easy, and may even be hard. Doing it well would likely require a wide-reaching effort the likes of which would eventually reach my ears, and the fact that I haven't heard of such a thing implies to me that it's probably not an AWS priority.
That said, your losing the forest for the trees, the accuracy isn’t that important. You can still bill for actual usage. A 15 minute granularity is vastly better than a 30 day one. As long as you can kill processes you don’t need to check in the middle of every action. Things being asynchronous is just the cost of business at scale.