I think when SSD drives become cheaper, Hard Drive companies may stop spending money on inventing larger storage size hard drive like how people stop inventing larger size floppy drives when USB drives got invented.
I feel most people will rather spend sloghtky more money for a 2 TB drive, or slightly less money for a 1TB drive,
I bet, 1.5 TB and other weird capacities like 700GB and 320GB drivea dont sell as many drives as 500GB, 2TB, 1TB,ETC.