In fact, I guarantee if someone made a hard drive with a controller with an mSATA slot for adding a SSD and offered the controller to be setup as pass-through (act as two drives) or caching (SSD keeps a cache of platter), it would sell like crazy.
This already exists. It's called Intel Smart Response Technology and it's been available on motherboards since it was release three years ago in 2011. It works with both mSATA and standard SATA attached drives.
Most people just haven't paid the extra $100-200 to buy an SSD to cache their HD not to mention the technical know-how required to install the drive and setup the BIOS and software correctly.
What we need is systems that are sold with this already built in so users don't have to do it manually. But that adds $100-200 cost to the system and many shoppers look at the price tag first when they're buying a new computer