However, you can't boot off them, and they don't build anything to facilitate their usage as a boot drive, even though it's perfect for use-cases like these where the machine is ephemeral and its local drive doesn't actually matter.

They should have a feature where you can provide it an S3 URL or EBS snapshot when launching an instance, and the control plane would write it to the local ephemeral direct-attach volume on launch.

Currently if you wanted to do that you'd have to roll your own with a minimal EBS as your root volume containing iPXE or some other minimal OS that is capable of formatting the direct-attach NVME and populating it before passing control to it (via kexec/etc?).

But I guess since EBS incurs additional cost, there's no incentive for them to make it easier to move away. I bet that in aggregate they're making quite good money off mostly unnecessary EBS volumes that don't actually contain any useful data.