To me the only point that needs to be made, is that all these sites, like twitter, reddit, and digg, were able to move to a nosql solution like cassandra at lit

To me the only point that needs to be made, is that all these sites, like twitter, reddit, and digg, were able to move to a nosql solution like cassandra at little cost, and this solution was able to handle the loads that these sites required. Plain and simple there was no need for them to move to ssd's, or buy their own hardware, or to perfect indexes... the main reason MySql worked for so many startups is because it was free, and the initial headaches for scaling the database have already been solved, and we already know the limitations, no need to figure out how to make it scale at any great cost. Yeah its probably possible for Dennis Forbes to figure out how to make a rdbs scale, but does that mean every startup has to hire him.