In my humble opinion, there is A LOT of wasted money in making datacenter machines easy to service. We create a little spreadsheet (it isn't rocket science) which includes the employee salaries taking into account different designs and how long it takes to open up and service a pod. For example, to access the Backblaze Pod's hard drives you must pull the pod out like a drawer and open the top. Many servers you can access all the hard drives from the front, without moving the server. Accessing drives from the front is much faster -> but you lose 2/3 of the density!! We pay $600 per month for a cabinet, just for the physical space. We can stack 8 pods in that cabinet, or about $75 per pod per month in space rental. So buying servers that are 1/3 as "dense" but easier to access drives from the front costs Backblaze $50/pod/month or $600/pod/year. We open a particular pod and replace a failed drive or fix some other problem maybe once every 6 months-> so you are paying $300 PER INCIDENT if you have front mounted drives that save the technician maybe 10 minutes of time. The math simply doesn't make any sense at scale.
I think a lot of the inefficiency is because the datacenter employees recommend these "nice to haves" to their managers to make the datacenter employees lives easier. But their company pays dearly for not doing the math. This isn't important if your company is massively profitable or if it isn't a "datacenter centric" company like Backblaze. But for us, it's the difference between life and death. Remember, we only charge $5/month and we don't have any deep pockets so we have to stay profitable -> there isn't a lot of margin in there to be wasteful.
Obviously that setup will run you more than yours, but I'd be surprised if the delta was more than 1k once you subbed in your consumer mb, controllers and backplanes.
That said, one small nitpick: I'm fairly sure Facebook does less revenue/user than you do: They have a billion users, and did (order of magnitude) 5 billion last year: That's $5/user/year. Which you do per month.
Still, it's amazing you can provide this unlimited service for that small fee!
We have done exactly what you mention -> migrated off a pod and then disassembling the pod and using the older drives as replacements. Hard drives often come with a 3 year warranty, so for the first 3 years it is free to get replacement from the manufacturer if they fail. But after 3 years we have to pay out of our own pocket to replace the drives which can change the cost/math a little.