So the point is that few people read old non-fiction books for their original purpose (i.e., 'convey knowledge'), but only for secondary reasons -- any useful observations the originals made would have been observed in newer, clearer works. In general, I agree with that.

But are there any exceptions?

Depending on what is called 'old'... I found Einsteins introduction to relativity one of the best layman's introductions.

Much older, I would say that Plato's/Aristotle's writings on philosophy are much clearer than the philosophy of the last centuries. They are misguided in various ways, but at least that is clear - and I wouldn't hesitate to recommend someone to read those works to gain some insight in philosophy, not just for their historical importance.