Could we arrive at a situation where all possible questions concerning ubuntu would have been asked and the corresponding answers given? The questions that would not have been answered are those that are either poorly asked or are duplicates?
No. New versions keep coming out and bringing with them new problems. Also, the set of possible tasks one might want to use their OS for is essentially infinite. Yes, we can reach a point where all the obvious, classic questions have been asked but the set of on topic questions is, for all intents and purposes, infinite.