The question seems to have at least two facets.

1. "Should be" as in "should any text be written in order to be relevant for a long time?".

This puts the emphasis on the writer of the text to make it so that the text answers questions in the far future. Imagine the scribe writing the Old Testament on the parchment and thinking about how to answer questions about DNA or the nuclear fusion. I think we know the answer to this, we don't need to look further the endorsment of slavery in the Bible did not stand the test of reason.

We don't seem to be able to predicting the longterm future with any reasoanble certainty, so I would say any text could be relevant for a longer time, while the issue addressed in the text is relevant and the answer provided is acceptable. But there seem to be no reason why it should be.


2. "Should be" as in "should we today consider it relevant?". 

Insofar as the issues addressed in the text and the answers given are relevant to the current times, and not further. The relevancy is not decided by our wishful thinking but by the text's relationship to the current issues. So it either de facto is relevant, or it isn't. But there is no necessity that it should be. That would mean putting outdated issues or outdated answers on life support.