It's not pre-computing an answer, it's training a neural network that will be able to answer that and similar questions with minimal or no human interaction.

Whether that works out or not remains to be seen, but it's definitely worth investing research effort in to.

with 100s millions of user, each question will become a common question.

Questions like: what, why, when, how? They are very common, but don't have definite answers.

The problem is, even with 100s of millions (or even billions), there are still an infinite number of questions that can be asked. Each of those questions can't be common.

Classic Pareto setup (https://en.wikipedia.org/wiki/Pareto_principle)