It's about fitting your utilization to the model that best serves you.

If you can keep 4 "Java boxes" fed with work 80%+ of the time, then sure EC2 is a good fit.

We do a lot of batch processing and save money over having EC2 boxes always on. Sure we could probably pinch some more pennies if we managed the EC2 box uptime and figured out mechanisms for load balancing the batches... But that's engineering time we just don't really care to spend when ECS nets us most of the savings advantage and is simple to reason about and use.