It would be understandable if lower Manhattan simply up and quit. Its status as the center of the financial world and corporate America itself has been on the wane for half a century. Then it was hit with 9/11, a credit crisis that laid Wall Street flat, and a long, brutal recession. Finally, the real estate market at the southern tip of the island is showing signs of exhaustion. The office vacancy rate has increased to 8.1% from 7.7% at the end of last year, according to CBRE-Econometic Advisors. As 4.4% square feet of new office space associated with the rebuilding of the World Trade Center site come on line in 2013, the vacancy rate in the city’s oldest neighborhood could hit 14%, according to Robert McGrath of CBRE. The surprising thing is, the experts view the coming dip as just one more cycle in the up-and-down history of the area. They are continuing to plan and to build, so that they can be ready for the next leg up, which they expect to occur in a few years. Kenneth McCarthy, head of Cushman & Wakefield’s New York research unit, stresses that the “important thing to remember is that downtown is still a vital market that will once again become, one of the best markets in the United States.”