Not really. SQL DBs can handle a crapload of traffic. Maybe not a million all at once by default, but generally with a million users you're looking at << 50k on

Not really. SQL DBs can handle a crapload of traffic. Maybe not a million all at once by default, but generally with a million users you're looking at << 50k on site at any given time, and if you split reads off to replicas you can handle a lot of scale. In my experience, 50-100k qps (writes) is where SQL starts to get especially hard