MarketWatch columnist and money manager Jon Markman says that while many investors don’t believe it, the rally still has plenty of room to run — and that corporations will keep it rolling even if individual investors don’t. Markman points to the government’s massive stimulus plan, record low interest rates, and the large amounts of money being pumped into corporate bonds as reasons for his optimism. “If the public doesn’t get on board with this bull market, the companies are going to do it themselves, both by buying other companies … and buying back their own stock,” he says. He thinks the S&P 500 could reach 3,000 by the end of this decade, but adds that it won’t be a smooth ride — there are enough economic troubles that the index could fall to 800 or so before turning upward.