Storage hardware startup here. We can do 72 drives in 4U for standard racks and 120 drives in 4U for deep racks, but it is difficult to service so we're only pushing to early adopters. However high-density servicability can be addressed. Our next system should have same drive numbers in 5U, much easier to swap drives. Not much can be done about the shipping problem, ha ha.
I'm working on something more similar to the Backblaze Storage Pod (in fact, originally based from their 2.0 design) for my employer. We played around with the hardware from their set up and decided to go a different route. It looks like we'll be getting 48 drives in 4U, but I'm sure we'll have some teething issues as we start to get our first few boxes up & running. 72 drives in 4U sounds like a lot of fun- I'm guessing SuperMicro SAS backplanes. Is there room in the box to make it self-sufficient or is it JBOD?