If I was running 10,000 transfer jobs in parallel and the longest of them took 12 minutes, the job would take 12 minutes

Yes but you are still charged for 18 hours of compute time?

That’s true, the cost needs to be factored into the model. But the near infinite bandwidth scalability allows the service to exist to begin with. If every job saturates your up and down bandwidth and takes 10 minutes, and you have 100 coming in a minute, you would need to design a ridiculous architecture that could spin up and down instances and handle queuing on the scale of thousands based on demand. Or you can write a simple lambda function that can be triggered from the AWS sdk and let their infrastructure handle the headache. I’m sure a home grown solution will become more cost effective at a massive scale but lambda fits the bill for a small/medium project