Well, like you said, it really depends on what you'll be doing. Granted, there are different paths one's trajectory will take, and you won't know them in advance.
On the other hand though, I wouldn't presume to write my own cryptographic protocol without having the fundamentals myself. Wouldn't you say that it's actually reasonable to learn this on your own? There are so many options, paid, or free, that can help you with this. You could take a Coursera class, or follow an open source curriculum like someone said in the comments down below.
You did say "most individuals" struggle to learn in such a way, but the argument can be made that individuals studying cryptographic principles are not "most people", which, if not intelligence, shows a special kind of perseverence and dedication that will also differentiate them in self-study. I wouldn't say what you're saying is immediately obvious.