I don't know, I found the article to be too bearish, bordering alarmist. Here's the thing:
1. The article is basing its findings on the market index. While that's not inappropriate, and it's true we're in a down market, but the stock market is a vast arena filled with plenty of exceptions. Some stocks are resistant to down markets, while others can thrive in it.
2. Long term investors should also remember that the market is cyclical. Chances are good there will be many more like these, along with the ups, and in the end, it may not even matter much.
3. The article is also basing on the performance of the past several years, and all together now: Past performance is no guarantee of future performance.

If anything, the fact that we have been in a down market has finally sunk in, and I believe that we're actually in the midst of a correction now. That's a good thing.
4. And finally, as sweeps pointed out, the best times to buy are in down markets. That's when bargain buys are typically at its most plentiful.
I agree with the evidence, but not the conclusion.