This is a common misunderstanding. At an enterprise level, share buybacks and dividends accomplish the exact same transfer of value to shareholders. In the case of dividends, shareholders who prefer buybacks will use dividend funds to purchase shares (and increase their percentage equity in the Nu network). In the case of share buybacks, shareholders who prefer dividends will sell some of their shares at the higher prices.
There is no difference in the amount of funds “leaving the system”. If there were, we would see corporations exclusively use one method or another. After all, corporations strive to maximize the return on investment for shareholders.