As the old saying goes "if I had 3 days to cut down a tree, I'd spend the first 2.5 days sharpening my axe". Sure, but at some point you have to actually start chopping. By analogy, at some point you have to quit worrying about fundamentals and learn the "stuff" you need to actually build systems in the real world.

By and large I'm in favor of spending a lot of time on fundamentals, and being able to reason things out from first principles. And when I was younger, I thought that was enough. But the longer I do this stuff, and the larger the field grows, the more I have to concede that, for some people, some of the time, it's a smart tradeoff to spend more of their time on the "get stuff done" stuff.