This seems like a lot of work just to keep the files on the EBS volume up to date
It is a lot of work. But the problem is not the amount of work. It's that you're asking for more than just keeping files on an EBS volume up-to-date. Your asking for updates on your EC2 instance to persist onto other EC2 instances.
EBS volumes are like your hard drive in your local computer. AWS does some work behind the scenes to ensure the data is replicated, but that's all within the confines of a single EBS volume.
Terminating an EC2 instance and deleting the volume is like me coming to your house and smashing your computer with a sledgehammer. Do you expect your data to magically replicate to your new computer without having first done something to protect that data? Of course not.
If an EBS volume is deleted, the data is gone. AWS does not magically replicate the data elsewhere. That's up to you, if you need it beyond the lifetime of the EBS volume. And that's a key point: not everyone needs the data to persist beyond the lifetime of an EBS volume.
When you turn off the "Delete on Terminate" flag for an EBS volume, it simply means that when the EC2 instance is terminated, the EBS volume is not deleted. When this happens, that EBS volume will sit unused in your AWS account.
The unattached EBS volume can be attached to another EC2 instance so you can use it, but again, that's not magically done for you. You need to do it yourself.
- ensures your data is as protected as it can, where it can, and
- provided the tools for you to go further.
Know where #1 stops and where you need to take over with #2.
So yes, preserving the files on an EBS volume is up to AWS. However, again, that's not what you want. You want more.
If you want the data to persist after an EC2 instance is terminated, then you need to do more. Creating an AMI image and updating your Auto Scaling group's Launch Configuration is what you need to do.