Depending on their workloads, running their own datacenter(s) might save them millions a month. I know it does in my own case. That said you give up flexibility for the $ savings. They may think the additional flexibility is worth the cost differential at this point.
Would be good to hear their perspective on this.
I believe we expect moving to the cloud to be more expensive than running our own DCs as you suggest, but I don't believe that takes into account any 'wasted developer time' you might factor into this.
I believe we started building this platform when AWS was very new, and hadn't seen a compelling reason to transition from it to the cloud until now. There's a couple of posts with more details behind our decision to go to GCP, but primarily it was to leverage their data tooling.
One has to wonder why they would opt for this. The entire story is a textbook example of where using a cloud would have been immensely better. Instead of leveraging mature pubic cloud offerings, they chose a path that evidently required huge amounts of developer time and resulted in a tremendous amount of pain/wasted time for downstream developers, only to scrap it in the end when they finally realized there's no point in trying to re-implement AWS/GCE. Think clouds are expensive? I'd love to quantify the number of wasted developer-hours resulting from this decision to use physical servers and see how it would stack up against even a very expensive AWS bill.