You wrote "You’d need to have every single service you depend on, including 3rd party services, on multi tenancy.". This is highly incorrect. I worked at several companies that have a multi tenancy strategy. It is:

* Automated. * Scoped to business critical services. Typically not including many of the 3rd party services. * Uses data replication, which is a feature in any modern cloud. * Load balancing, by DNS basically for free or a real LB somewhere on the edge.

If you fail at this you probably fail at disaster recovery too or any good practice on how to run things in the cloud. Most likely because of very poor architecture.

Any 3rd party service that you depend on would need to be multi tenancy because if their one node fails, your service goes down too.

It's easy to replicate multi-tenancy for RDS since it's built in. But it's not cheap. It's double, triple the price.