There's no one cutoff or threshold at which hard drives die. SSDs have already made hard drives smaller than about 320GB completely uneconomical on a pure $/GB basis, and have killed off high-RPM hard drives. SSDs will continue to displace hard drives anywhere that performance or battery life matters.

You can break down drive costs somewhat into the fixed costs (SSD controller, or hard drive spindle motor and actuators) and the costs that vary with capacity (NAND or platters+heads). The fixed costs tend to be lower for SSDs (or at least SATA SSDs), and the variable costs are higher for SSDs because adding NAND is more expensive than another platter.

Hard drives smaller than several TB are no longer getting new technology (eg. you won't find a 2-platter helium drive), so whenever NAND gets cheaper the threshold capacity below which hard drives don't make sense moves upward.

For the near future, hard drive manufacturers have a clear path to outrun the capacities available from cheap SSDs. NAND flash gets you more bits per mm^2 than a hard drive platter, but platters are far cheaper per mm^2—enough to also be cheaper per bit. I think that relationship will still be true by the time hard drives are using bit-patterned media and 3D NAND is at several hundred layers.

Ultimately, it might make more sense to ask when we will see SSDs having taken over the former hard drive market with hard drives having moved entirely into the market traditionally occupied by tape.