I agree with the OP - make a full-size 5.25" HD, and/or even make a 5.25" double-height drive - just keep the SATA and power connectors in the same relative position so they'll slide in the Mac Pro's drive bays .
Lots of space then for more and bigger platters without the never-ending race for greater areal density, and the associated problems with errors and head positioning. Perhaps (as the OP says) they could also run slower and quieter without any loss of performance, and reliability would increase?
I can actually remember the first hard drives - referred to back then as 'Winchester' - which were 5.25" and double-height. Hitachi made one in 1989 with 8 platters at 3600rpm. I've always wondered why this form factor isn't still manufactured as an option for desktop users who need lots of space.
As for sales - I'm sure if you built it, the people would come and the manufacturers would then adapt their casings. The RAID and NAS market segment would probably love big, fast, quiet and more reliable drives.