The thinking behind "as long as people think it will crash, it won't crash" is as follows...

1) most people thinking that a crash is near means that they view the stock market to be overpriced.

2) this means they won't buy as much and inflate prices even further. in fact, many may start to sell.

3) this will tend to limit price increases and may even depress the markets a bit

4) Thus, while there may be a decline if economic conditions turn sour, there's a lower likelihood of a "crash" (large drop from current prices).