If you think StackOverflow is the epitome of scale, then your view of the world is somewhat limited.
I worked for a flash sale site in 2008 that had to handle 3 million users, all trying to connect to your site simultaneously to buy a minimal supply of inventory. After 15 minutes of peak scale, traffic will scale back down by 80-90%. I am pretty sure StackOverflow never had to deal with such a problem.
a bit more context: High-velocity transactional systems (e.g any e-commerce store with millions of users all trying to shop at the same time), I helped to build such a 10 years ago here is the presentation - https://qconlondon.com/london-2010/qconlondon.com/dl/qcon-lo...