What I mean is, instead of writing about 8TB disks vs 2TB disks, get hard numbers for those and calculate costs for different versions to the costs in the cloud.
Instead of saying that engineers are expensive, look at market data to determine how much you have to pay to hire these people.
Instead of writing about the potential of full cabinets, get quotes from data center providers of what they can provide and how they would deal with this. I'm sure Equinix has solutions for that kind of problem.
And in addition to that, it seems that managed dedicated wasn't considered at all. Why hire someone to swap disks for you? Why not just renting capacity for servers and have the data center provider taking care of hardware issues? They operate at bigger scale and can do this with little overhead costs. You're still much cheaper than the cloud but don't need to drive somewhere to swap a server (you just switch to a spare one until the data center fixed the problem). With Gitlab's scale, it shouldn't be too hard to get a decent bespoke quote from companies like OVH.