Localstack is great but I wish it wasn't needed. Engineers have become too comfortable stringing cloud services together with minimal code in a way that makes them unable to be run locally. This is also a hard problem to fix after the fact. If you make a mock of the cloud service, how do you know you captured all of the functionality? How do you know your mock won't pass all of the tests, but the real cloud server will behave differently and break production? Greenfield you can anticipate this and explicitly test all behavior locally.