In herokus case:

Their "dynos" are ephemeral. They could literally deploy the images to a backup environment hosted elsewhere. Their data services could all be synchronously replicated to that backup environment. And thats it - they dont offer any other core services (and their other services run on the same platform.)

So for (at most) double their infrastructure cost they have another network they can immediately switch over to.

And herokus already soooo expensive. Even if you used a 1-to-1 mapping for ec2 to heroku dynos (which they dont - its multiple dynos per backing instance), you would be looking at 5-10x markup using on-demand instances! Reserved instances are even less expensive. Spot instances can be 5x less again!

I think they could retain their current pricing model and still offer this kind of resiliency - at a minimum.

Fly.io is making strides in this direction, distributing the VMs across multiple availability zones, and routing traffic internally from their multiple geographically distributed POPs - but you need to roll your own DB VMs for multi-az synchronization..

EDIT: seems they do provide managed postgres with synchronized replication now (in beta), neat!