Is that just Silicon Valley though? I'd wager there are just as many companies out there trying to support a legacy code base or just doing straight up SharePoint work than worrying about the latest and greatest flavour of the month.

Exactly. The flip side of the constant churn is the majority of companies have code bases that are not the flavour of the month that they need to support. Given enough time, most companies will end up with multiple generations of flavours of the month that require attention.