Since 1926 the S&P 500 has returned an average 10.4 percent per year. As a harbinger of the future this number must be taken with a certain caution: Much of the post-1926 return was from high dividends (averaging 4 percent), while today's yield is only 1.7 percent. Some of the past return was from expansion in price/earnings multiples, an expansion that cannot be counted on to continue. Still, it is not unreasonable to expect an 8 percent annual return from stocks over the next 30 years, or perhaps 7 percent after taxes. You must be prepared to accept lots of uncertainty in your annual returns and some uncertainty in the long-term return. It is quite conceivable that stocks, while you hold them, will return less than your mortgage costs.
Which is just another reason why I am attracted to the idea of paying off my mortgage sooner rather than later.