In my view one of the puzzles of physics is actually its success over the past couple of centuries. That we have such precise and comprehensive models of anything could not have been anticipated.

But this success doesn't guarantee future success or a predictable rate of future progress. Whenever we agree to work on a fundamentally new question, we are never sure whether the answer will arrive in 5 minutes or 5 centuries. Reconciling quantum mechanics and gravity might be a 5 century problem for all we know.

Likewise, we can never claim with assurance that any non-trivial question is un-answerable. When problems get hard, books get written about the boundaries of science.

As I understand things, it took humanity some 1000 years to solve quadratic equations.