Missed the point
SSDs may be heading into a good density to catch up with HDDs, but your assumption assumes HDDs stand still for the next 4/5 years. Common SSDs of the 1TB flavor (if they get here in 4 years) will be matching up against 1TB/sq in drives that float around 6TB+, and likely the spindle drives will STILL be cheaper.
On a side note, a 5.25" HDD would be an interesting option. Currently cramming an extra platter in a drive to make a jump in drive size could be side-stepped by making a 5.25" casing, and perhaps adding even 1/4" radius on the platters and maintain the 5400rpm speed. Perhaps add a 1/2" radius and lower to 4900rpm. Another alternative could be TWO spindles of a 2.5" flavor interweaved with an arm per spindle. In a 5.25" drive the size of a CD-ROM it might be possible. Only problem could be the proximity of the spinning platters to each other in the interwoven part. Could make the drive negative pressured to reduce/remove problems related to air. Who knows?