That's a valid point and a good solution, you could easily replace the ECS with AWS Lambda for running your task without having idle machines, but the scheduler

That's a valid point and a good solution, you could easily replace the ECS with AWS Lambda for running your task without having idle machines, but the scheduler is still necessary. If you have more complex tasks or are using languages not supported by Lambda, ECS can still be a very cheap way around (you can use a t2.micro instance and share it across all of your jobs)