But this thread was initially about a dev running some experiments and getting a $200 unexpected bill. They would have been very happy with a $30 cap that just deleted everything. That functionality would be easy to build if they wanted to. But they don't. For other reasons, not because it's hard.
The thing I dont buy is Amazon claiming its too hard to build a cap. A cap that suits some people would be easy. What they really mean is... A simple hard cap is only useful to customers we dont care about because they dont pay us enough. An advanced cap with all the kinds of failover options and thresholds that a medium sized business might want is complex and the people who actually pay the big money (those we care about) don't actually want caps anyway.