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This question was not answered by the Buddha not because of "there is no one to witness it".
The reason is something else, according to the Commentary.
The question was asked from a position that believes in a "Person". The questioner thinks there is a person called Thatagata and ask what will happen to that person.
In Theravada there is no any real entity of Person or Self. There are only Phenomena.
If the Buddha answered as "tathagata does not exist after death", then the questioner would understand it as "the person (self of) tathagata does not exist after death".
If the Buddha answered as "there is no real entity called person", then it is not an answer to the question. The Omniscient Ones talk to the point and don't provide answers in whatsoever invalid way.
There is a lesson series by a renowned expert in Abhidhamma which clarifies many of such issues: (Fundamentals of Theravada Doctrine)