I have never had a production issue in other regions due to a us-east-1 outage. The worst that ever happened was I had to wait to update a Cloudfront distribution because the control plane (based in us-east-1) was down, but the existing configuration continued working fine throughout.

I don't know what the architecture of IAM looks like, but somehow it's never suffered a global outage.

AWS is really, really good at regional isolation.