Should Palin not run, she will have absolutely no, none, nada "power" to effect politics,or anything else, in the future. And no future GOP president will ever nominate her for any position, just to have her dominate the conversation in the media. So,for her, it is now or never.
As for whether it's now or never, they said that about Reagan in 1976, or Nixon in 1962.
The point is, a leader won't be able much longer to take swipes from Fox News, as, say, Huck does, or post pithy sayings on FB like Palin does. Sooner or later, "THE" person is going to have to get in front of some of these issues.