In addition to my experience as a student in a bunch of mathy disciplines, I taught math for a few years. I know it's just an example, but I have not seen very many people who make it usefully far in mathematics through force of will.

I certainly agree with you about how transformative mathematical study is in how we process seemingly unrelated topics. My doubt is that it would be as transformative for people who had to buckle down and will their way through the material.

Like you suggest, people draw those transformative lessons from many places. My expectation is that looking for them in subjects that you find inspiring or interesting will be more fruitful. On that note, though, I do think it's important to give things a chance and not just assume there's nothing in it of value.

I certainly think this isn't a topic where one approach is correct and the other is incorrect. It's more that I think it more often makes sense to focus on your strengths, because that moves you into less dense parts of the skill distribution, which allows you to charge more of a premium.

The reality is that you should be evaluating how the benefit of learning something compares to the cost. Sometimes, that will mean doing something very costly, because it's a bottleneck for a bunch of other stuff you are good at.