It's a valid point. If us-east-1 had a big, prolonged, outage then there would be a huge rush elsewhere and nobody knows how AWS would cope in that scenario, especially for the niche services that are only available in a few regions.
Two thirds seems high though. It also doesn't fit with other published stats about AWS AZ sizes, in theory us-east-1 can be at most 5 times us-west or eu-west.
Biting the hand that feeds IT
