The question was asked because over the past three years nearly every stock that he bought simply rose. And kept rising. We agreed in the end that it wouldn't all end badly, but that it certainly couldn't continue like this indefinitely. But no, the market won't perform horribly simply because it has performed so well for the past twenty years. The market has no memory and no idea how it has performed, nor does that matter.
Stocks move with the economy and on earnings growth.
If both continue to be favorable for an eternity, the market would very gladly continue to grow indefinitely, for an eternity. But there are cycles. Economies expand for decades and then finally slow or contract for a time. And stocks go with them. There are cycles that affect entire generations.