> 1 million IOPS on a NoSQL database

I have gone well beyond this figure by doing clever tricks in software and batching multiple transactions into IO blocks where feasible. If your average transaction is substantially smaller than the IO block size, then you are probably leaving a lot of throughput on the table.

The point I am trying to make is that even if you think "One Big Server" might have issues down the road, there are always some optimizations that can be made. Have some faith in the vertical.

This path has worked out really well for us over the last ~decade. New employees can pick things up much more quickly when you don't have to show them the equivalent of a nuclear reactor CAD drawing to get started.