When Caterpillar Inc.’s chief executive officer raised the possibility on Tuesday that now could be as good as it gets, he awoke his investors and the broader market to a concern that has been nagging some of the smartest money managers for years. The issue is that the average net profit margin of the companies in the S&P 500 Index is expected to hit just above 11 percent in the first quarter. If profit margins are not only high but at their ceiling, which is what Caterpillar’s CEO seemed to be suggesting, that would be a serious problem for the market and perhaps why — along with the fact that 10-year Treasury yields are pushing 3 percent for the first time in years —investors reacted so harshly.