Earlier quoted context omitted.
It being a certainty is wrong, but there is a huge differential between the growth of total market value and the lackluster fundamentals, GDP growth, and jobs numbers. To put another way, there's a lot of "potential energy" being built up in the markets right now. That doesn't necessarily mean they'll pop like a bubble - but there's really no precedent for them to continue rising.
6 months ago you could accurately say there's "no precedent for them to continue rising". You may be saying the same thing 6 months from now.
It could be true that the market can continue to grow well past 15% above fundamentals for the indefinite future. But the more it goes up, the more improbable it becomes.
If you've flipped heads three times in a row, you're right that I would look foolish saying the next one can't be heads. But at the same time you cannot keep flipping heads forever.