Anyone knows why John Bogle said the good times will no longer roll in his book?

There's a limit to how much something can grow exponentially.

Imagine two companies, one with 100 customers and another with 100,000,000. Which of the two do you think can grow its customer base at 10% annually? The larger company at most can grow for 50 years or less before the world runs out of potential customers. Not to mention, there are also other factors such as lower fertility and ageing population.