How big is the gap between Mandarin and ancient Chinese?

Mandarin is not completely equivalent to ancient Chinese, because the way people speak, the great migration of population and the integration of dialects in various places have made great changes in the pronunciation of Chinese characters for thousands of years. For example, "Rusheng" has disappeared in most dialects, and "Jiantuan" has also disappeared, while "Er Hua" has appeared in some dialects. Therefore, some people think that reading Tang poetry in Mandarin will lose the charm of the ancients.