Re: We need more reliable, not just larger.
Taller drives = higher spec motors and more platter flutter. This won't happen and larger platters won't happen for similar reasons.
Fly heights are 1/10 what they were when drives broke the 100Gb barrier. There's simply not the engineering tolerances available to allow taller drives/larger platters, even if drives were slowed to 3700 or 4200rpm again.