all dinosaur species were very old

On a human-scale timeframe? Yeah, sure. On evolutionary timescales? According to a PDF by “Charles Sibley” whose content I can only see a small Google snippet of, “Galliformes and Craciformes diverged at a 21.6.” (I assume that’s MYA) And certainly a lot more diversification has happened since then. Gallus domesticus is probably less than 8,000 years old.

Though a new sauropod would certainly be quite the marvel.