I will be very surprised if Palin gets a second try.
Reagan was running against a standing president. Nixon came very close to beating JFK, and could not get an endorsement from Eisenhower in 1960.
Bob Dole was not offered a second try, and it seems inconceivable that McCain will be offered one, either.
Others who did not get a second try were Willkie, Landon and Dewey. Poor Bob Taft thought he had it all sewed up until they got Eisenhower to run.
Palin is unlikely to become more astute over the years, I would say that her popularity is near its peak.