I never understood why AWS doesn't provide something like LocalStack out of the box.
Any team building serious software on AWS needs to mock AWS services in their CI/CD pipelines. What exactly are they expecting developers to do? They would probably argue something like "spin up real infrastructure so you are as close to production as possible" because this way they could make even more money while also avoiding the implementation / maintenance cost of the mocks.
That doesn't make sense. You use AWS services because they are in the cloud, not because you want to do self-hosting. Similar to Cloudflare wrangler, localstack should be just a well-built local mocking service.
If you're a cost-conscious startup, you could use AWS Local Thingie to run it on a 5€/month VPS while you validate your idea.
Then if you get actual customers you could move it to proper AWS - or you might find out that Fake-AWS is actually enough for you and a 20€ VPS is enough for your uses and pay 0€ to Amazon, while still filing bug tickets to AWS Local Thingie Github for bugs and feature requests =)