"Tunable spending limit" has consequences that can create other, equally real, problems.
Best effort: Turn off all compute resources, drop dynamically-adjustable persistent resources to their minimums (e.g. dynamo write and read capacity of 1 on every table), leave EBS volumes and S3 alone. In some cases, a user might find their business effectively offline while still racking up a massive AWS bill.
Hard cutoff: Very close to deleting an AWS account. In addition to compute and dynamically-adjustable resources to minimums, this means deleting S3 buckets, Dynamo tables, EBS volumes and snapshots, and everything else that racks up cost by the hour.
The best effort approach sounds reasonable to me. The hard cutoff solution sounds worse than the problem it purports to solve.
Agreed that AWS is poorly incentivized to fix the problem.
What semantics do you expect when your spending limit is hit?
Turning off all compute resources (EC2, Lambda, Fargate, etc) seems obvious, but what about systems managing state like S3, EBS, and DynamoDB? Should buckets, volumes, and tables be deleted?