Not exactly. If you stick to EC2 or containers with general Linux for your servers, and RDS Postgres and Mysql for DBs, and similarly for other services, you supposedly would be able to move to another cloud without issue.

We thought we'd be okay.

But when AWS started getting too expensive and we began looking to migrate, we discovered that there was a lot of 'glue' in our stack using bits and pieces of AWS-specific services. A continuous integration was using S3 buckets and Lambda, a few clever devOps guys had found ways to create alarms with SNS and SQS.

So while 90% of it was just a matter of moving to general CentOS containers on another platform and spinning up new Postgres and MySql and Mongo instances, that 10% of AWS specific code still causes issues that had to be remedied.

If I were in charge of a large infrastructure on AWS, the first thing I'd do is absolutely lock down every service that wasn't explicitly whitelisted, and open them as needed. Not wait till half your operation depends on dozens of AWS services that, while make things easy initially, lock you in to their platform.