I think if you expect to be under 60K/s and suddenly find yourself at 20K/s heading for 200K/s-- you have a better problem than if you built for 1M/s and actual load is 20K/s. The unexpected success of the former will pay for a lot of band-aids and scaling, while you're pretty stuck with the cost structure and upfront spent capital in the latter.

IMO one should design for actual anticipated scale with moderate margin, only exceeding this when it's relatively "free" to do so. (If you can buy bigger hardware for a few K, or if solutions are equivalent other than scalability, pick the bigger solution).