It's true that differentiation is a factor in career longevity, but it's not the point I was trying to make, nor the one that I would emphasize. The true advantage of learning how to do kernel development (over, say, a Javascript framework) is mostly that you're learning a technology is likely to be in demand in 20 years.

Given how much JS development thrashes, it's a pretty reasonable bet that whatever you're learning this year will be out of fashion next year, and irrelevant in five years. That's the high-order bit, when you're thinking about your career.

Even for something as "narrow" as linux kernel development, I can pretty much guarantee that someone will want hire you in 20 years, so long as you are competent. I can't make that guarantee for most technologies.