You probably don't need to compel them. PRISM have shown it's easy to setup a program where they are forced to just share everything through a back door to the gov and not tell anyone.
They are willing to give you the only key, provided you release them from obligation to help you get your own information back if you lose your key.
For most people's threat model, this is not necessary, as even in the case of the San Bernardino terrorist iPhone, Apple doesn't tend to defeat their own security measures on demand. But following that situation and others, Apple added additional measures making it even more difficult for themselves.
See this support article:
"Advanced Data Protection for iCloud is an optional setting that offers our highest level of cloud data security. If you choose to enable Advanced Data Protection, your trusted devices will retain sole access to the encryption keys for the majority of your iCloud data, thereby protecting it using end-to-end encryption. Additional data protected includes iCloud Backup, Photos, Notes and more."
