SSDs are better than HDDs for GB/cm3 already, but consumers for the most part don't want to buy $8000 SSDs. (Large companies/data centers are already buying them as fast as they can be built).
Laptop (2.5") drives are limited to about 2TB, and they're 15mm thick. SSDs are hitting 2TB using a 7mm Z-height..
Really, the only play for HDDs is $/GB, which already has nearly evaporated at the low end when you consider all the other contributors to cost in a system.