It's a big data company doing an amount of traffic per day that exceeds what you can fit in RAM, including those fancy x1.32xl's with 2 TB RAM and a monthly cost of ~$6,000 for a reserved instance.

Yeah, I'm pretty sure we can't simply run it all from a single server.

And then the server goes down and you've got downtime. Or you want to release a new version and you have downtime (ie: no rolling upgrades). At some point you end up re-inventing enough of k8s to makes it sort of silly to not use the actual thing.