People kept up bringing this argument since the very beginning when people already asked for this feature. This used to be the most upvoted request on AWS forums with AWS officially acknowledging (back in 2007 IIRC), "We know it's important for you and are working on it". But they made a decision not to implement it.

The details don't matter, really. For those who decide to set up a hard cap and agree to its terms, there could be a grace period or not. In the end, all instances would be shut down and all data lost, just like in traditional services when you haven't paid your bill so you are no longer entitled to them, pure and simple.

They haven't implemented and never will because Amazon is a company that is obsessed with optimization. There is negative motivation to implement anything related to that.