I believe one is ready to make an original contribution when one understands the problem and also understands why their solution solves it.

Mathematics is meant to talk about ideas, abstractions, and truths over prestige or authority.

This topic often comes up when we stop thinking for ourselves and place mathematics in a sociological mindset. 

Instead of thinking "Why are we defining it this way? Why are the existing tools insufficient? Why does this proof work? Can I revise this theory to be more concise?", we begin to think "Man, this person wrote 1000 pages of mathematics, there's no way I could possibly understand it all." 

There is irony here: the less of some theory one understands, the more difficult it is to challenge. In many instances, one is indeed lacking key insights to fully understand the theory. But there will ALSO inevitably be instances where every member of the population assumes that some other member knows it better than they do, thus no single member attempts to challenge the theory.  Each chapter or argument may be optimized locally, but opportunities nonetheless exist in the global scope for huge simplifications.

It's important to be able to challenge the arguments. If a theory is so large and complex that amendments cannot be understood within the overall context of the theory, it is unlikely that such an amendment will be simplified or assimilated. Mathematical theories in such cases will usually undergo a process of explosion

The point being that having very large theories taking years of work to understand is all the more reason to believe that opportunities exist for contributions.