لَا يَثْبُتُ صِدْقُ الرَّأْيِ بِقِدَمِهِ، بَلْ بِدَلِيلِهِ.
The truth of a view is established not by its age but by its evidence.
The not-but pattern replaces an inadequate criterion with an evidential one. It is forceful against an audience that equates age with correctness, whereas a tentative learner may respond better to a softer invitation to inspect evidence.
A fitting expression is not simply the strongest-sounding one. Teaching, debate and public exhortation may need different registers even while preserving the same substantive claim.