If I can store drive encryption keys on a HSM in my old, consumer grade laptop, I would hope that large storage systems have at least the same degree of protection.
If I understand it correctly, the HSM on the HDD dying is about as likely as a HDD PCB failure. Of course in these scenarios you can't just swap the PCB's to recover the data, but in an Enterprise setting you would have mitigated this anyways, by using a form of redundant storage. If you rely on just one drive for your data's continued existence, you're doomed anyways.
I'm going to trust the storage manufacturers when they offer a secure erase function that it whole disk encrypts, and secure erase removed the decrypt keys everywhere they exist. It's a conversation you have when you establish the vendor, and they're the ones that own the risk (fiancial, reputational, etc) if it turns out the key is stored in plain text on ring 0, sector 0, disk 0 and someone talks about it at Defcon.
The point is: I'm tired of 'well what if?'...that comes up EVERY time there's a question about data destruction....'we should shred it "just to be sure"' is stupid.