>Less if you use lambda and api gateway.

There's the slippery slope into vendor lock-in via combinatorial explosion of complexity

what op means is that for low traffic or no traffic services, lambda scales to zero where ec2 and rds do not.

scale to zero is very good when zero usage is regularly reached.

"Run your own container" offerings also scale to zero, all clouds have something to that effect and it doesn't require you to embedded your logic into their tar pit.