Indeed, though that must have cost them twice as much to do the migration with the two systems being live in parallel. Some choose to avoid the extra cost.
The cost of doing a massive, irreversible migration that doesn’t work seamlessly the first time is significant. And often more significant in engineering time, lost revenue from services going offline, trust both from external users and internal teams watching the shitshow, and possibly data loss or backwards-incompatible changes than those server dollars.
It’s funny that one consistent theme of the best engineers I’ve worked with is less their “amazing 10x ideas” than the number of “sounds like a good idea if you’ve never done it” ideas that they shut down. Huge migrations are one.
I don’t know how they got there but that’s expensive, disrupts their supply chain, and I suspect they’d cheerfully pay four times as much not to have to slog through this mess.