A/ According to https://en.wikipedia.org/wiki/Knowledge_space theory, the path to greatness involves mastering join-irreducible concepts/skills.

Mediocrity can always be achieved via the join of two existing skills (if you know A and you know B, then doing C is "just" doing both A and B at the same time), but greatness requires leaps (if you know A, but the path from there to B doesn't involve any combination of other existing skills, you'll just have to try a bunch of stuff until the penny drops/inspiration strikes/the muse vists/etc.)

(being mediocre myself, I can't speak from personal experience, but this is certainly how it looks when regarding the acknowledged [Nobel/Olympic/Turing] greats with whom I have studied/trained/collaborated over the decades)

B/ Consider Niels Henrik Abel's opinion on whom to study.