I keep hearing this from people who don’t say they’ve built out Serverless stacks. But I keep hearing from people who switched that they’ve been able to cut costs. So in my limited experience this has so far proven not to be true in practice.

Going from autoscaling in EC2 (with a mix or RI for baseline load and spots for peak) to Lambdas has cut our service costs down by half. This is largely due to better utilization rates.

Under-utilized machines in people's stack.