I think we have different meanings of "idle". There's idle in the sense of "this resource is reserved but not in use" and idle in the sense of "this resource is not reserved and can be used elsewhere".
Most folk want cold starts for economic reasons and warm starts for performance reasons. In a hosted setting you can't truly set your policy, you can only try to encourage by external stimuli.
I still have these resources idle, when I use Riff.