This is the Innovator's Dilemma. Spinning mechanical hard disks will continue to get larger and larger, but they will be far above the requirements of most users. Some people will want a 4 TB hard drive for their laptop, but for most people 500 GB is more than enough.
SSDs will get larger and cheaper until they fit the average consumer's taste. Also, SSDs are faster and more energy efficient, so they will be providing additional benefits on top of size. Thus, my bet is that SSDs will probably move to replace spinning hard disks over the next 5-10 years.