StackExchange System Architecture
So they have 185 req/s for all their Stack Exchange sites. Let's say half of that is on Stack Overflow. That's 93 req/s and they do it with one DB (and one hot standby) with 384GB of RAM. I don't know many sites that big, so I think that the majority of people that say "the DB does not scale" work all for online properties with more that hundreds of millions of users, or are optimizing for a problem they will never have.