I didn't read it as saying "stick to one language for life", although even that isn't a bad plan in some cases - as long as you also learn other languages eventually. For instance, some folk have had very long C-centric careers and can still pull some of the highest paying programming jobs.

Despite having been forced to use a bunch of languages over the course of my career, you can only focus on so many. Truthfully, this is a great time to be a language dilettante, but one must pick and choose where to get serious. For instance, at the moment I'm focusing on Swift for a variety of reasons, but I wish I could spend more time with Scala and Julia. There are only so many hours in the day, and only so much mental bandwidth.