Your line (assuming you are describing a rising line and not a flat one - in which case we would have a dispute) would be helpful for one sort of investment strategy: "buy 'em and put 'em away". My strategy achieves earnings better than the market averages. It is based on the fact that the market does not follow a straight line but has peaks and valleys. There are bull and bear markets. If you are smart, you can avoid or minimalize the bears and utilize the bulls.

Also note that the worth of the market and of individual stocks depends on expected future earnings and not necessarily on what the companies are doing right at that minute. Right now expectations are that earnings will continue to rise for a while. That expectation shows up in market valuation - and it should. If you were going to buy a small business, wouldn't you pay more for one that you thought would make more money next year than it did in the current year?