No it won't be harder for the entertainment industry to create rootkits. You just buy a proper certificate and sign it like any other driver developer. Secure boot does nothing to protect the users from the entertainment industry.

And if we remember what happened the last time that happened when an entertainment company deployed a rootkit. They made it much easier for unskilled malware writers to hide their work. And IIRC it wasn't trivial to remove the rootkit Sony deployed.