Companies with fewer users could have scaling problems earlier if their load is very write-heavy, if they generate artificial load from things such as crawling, if they have stricter latency requirements...
Moreover, note that Redis server. Stack Exchange uses caching very agressively [1], it is not the relational database doing all the work.
All that being said, I don't disagree in general: most startups should stop worrying about how they will scale their DB and stick with simple solutions that work.
Companies with fewer users could have scaling problems earlier if their load is very write-heavy, if they generate artificial load from things such as crawling, if they have stricter latency requirements...
Moreover, note that Redis server. Stack Exchange uses caching very agressively [1], it is not the relational database doing all the work.
All that being said, I don't disagree in general: most startups should stop worrying about how they will scale their DB and stick with simple solutions that work.
[1] http://meta.stackexchange.com/a/69172