The thing about fitting everything in one rack, potentially, is vibration. There have been several studies into drive performance degredation from vibration, and there's noticeable impact in some scenarios. The Open Compute "Knox" design as used by Facebook spins drives up when needed, and then back down, though whether that's for vibration impact, I don't know (their cold storage use [0]).
0: https://datacenterfrontier.com/inside-facebooks-blu-ray-cold...
https://www.dtc.umn.edu/publications/reports/2005_08.pdf
https://digitalcommons.mtu.edu/cgi/viewcontent.cgi?article=1...