I'd be fascinated to hear how Terraform would have intelligently known that those instances were not meant to stay on overnight.

Honestly, I'd create the instances using an ASG, then set the ASG size to 0 (or throw inside a while loop until any errors go away). Always create instances from an AMI and always put them in an ASG (even if the ASG only has 1 item min, target, and max on it).

I love Terraform and ASGs but that still doesn't solve the fact that their SRE overprovisioned. They might have even used both things!