If you hadn't tethered your software to proprietary AWS services in the first place, you wouldn't have the problem to "replicate" all of that on your own infrastructure. You would simply "have" that stuff - or better said: you'd have the parts that you actually need (having a giant grab bag of ready-to-use cloud services at your disposal tends to entice developers to use them, regardless of actual need, especially in "enterprisey" environments where just another service doesn't really ring a bell with anyone, corporate pays for everything anyway and every developer is constantly engaged in putting more tech buzz words on their CVs).
Queueing and messaging systems, databases, key-value stores, backup solutions etc. have all been invented pre-AWS, and there are battle-tested solutions for all of that out there, actively being used by companies which did not choose to depend on AWS.
I run a business on AWS. I have a few terabytes of data on S3. It costs like $30/mo. I have about a half terabyte of data in RDS. That's about $40/mo, plus a couple thousand a year for the RIs. I run the company myself. I run SES for less than a dollar a month to handle all my transactional emails.
If I move this out of AWS, I need that data replicated for redundancy. I need some meaty servers set up, and I need to maintain the boxes that run the storage and database software. I need to build out the observability software that I get for almost free with Cloudwatch. My database instances need to failover appropriately, which doesn't happen automatically. I need to monitor disk usage and add capacity as the databases grow. I need to set up and manage whatever firewall setup I otherwise get for free with VPCs and security groups.
Managing my own email infrastructure is almost laughable. Spending more than about two and a half minutes on it each month outweighs the benefit of just running it on AWS.
My time isn't free. I either need to hire someone to build this, I need to learn it and apply it correctly, or find some magic docker stuff that does it for me.
Where is the cost savings of moving out of the cloud for me? If I'm spending all of my time in a SSH session installing kernel updates or diagnosing why my storage cluster is misbehaving, when am I supposed to actually build my product?
My company has all of these things on-prem, but with teams of at minimum 5 FTEs backing each one. That's the point here: a small shop cannot necessarily afford to fill an oncall roster with Kafka experts or Postgres experts or whatever else is needed.
Queueing and messaging systems, databases, key-value stores, backup solutions etc. have all been invented pre-AWS, and there are battle-tested solutions for all of that out there, actively being used by companies which did not choose to depend on AWS.