The reality is that Joe is correct, there must be a law of diminishing returns. Unless I am much mistaken ticket sales for the F1 Grand Prix at Silverstone are down this year and one reason given is the dominance of Red Bull although it is more complicated than that as the economy is still stuffed, no Brits expected to win, tyre issues etc.
However domination by one driver does seem to lead to a loss of interest in F1 and the same must be true for Le Mans.