Then as the OP said, AWS isn't for you if you have a constant 24/7 compute load. In my case the load is very bursty. I sometimes do millions of requests per day

Then as the OP said, AWS isn't for you if you have a constant 24/7 compute load. In my case the load is very bursty. I sometimes do millions of requests per day, but I typically don't. We lowered our TCO by 75% by moving from on-prem to AWS and using Lambda. It really comes down to your compute load profile.