A dead comment says:

> Of course, this assumes independent events. World Cup, super bowls, etc break these assumptions.

Yes, this is very true. The model here works for Poisson arrivals and exponential service time (the M/M), which are poor approximations of real-world traffic patterns (which tend to be non-stationary and non-ergodic, and include substantial seasonality). However, the frequency of that seasonality is typically rather low (e.g. daily cycles), and so these stronger assumptions are quite defensible for short time periods.

A better approach is to do simulation with real traffic patterns, or even with more sophisticated parametric models, and get better answers (e.g. https://stability-sim.systems/). The good news is that kind of simulation is cheaper to do than ever before.