Timegold

"Well, maybe it was, but the future isn't written in stone. We each make our own destiny." -Wes, Power Rangers Time Force

Sure, stones may erode, but they're relatively well preserved throughout eons of history - just consider the Rosetta Stone and what records it's maintained. But there's an obvious asymmetry between the past and the future, in that one is known and unalterable, while the other is uncertain and dynamic. From Wes's perspective, his course has yet to be set, and he still has the opportunity to change it.

In comes Time Force. Upon going back a millennium, the asymmetry is flipped, in that the past is in danger of being altered and their mission is to preserve the future. The Time Force Rangers seem to be stuck on their destinies, but Wes, who has little experience with time travel, doesn't see it that way. By telling them that their destinies are not set in stone, he inspired them to produce change, to not be bound by the rigid rules that the chain of command and the timestream have put in place. It's a line embedded with hope and a thematic message, it's why Wes rightfully deserved to be the focus character even without being the leader.

The corresponding line from Mirai Sentai Timeranger, "Even if we can't change the future, we can change our tomorrows," carries similar weight, although there's slightly more impact in that all the main characters have pasts to overcome and futures to be written. And there's some symbolism in that Tatsuya runs Tomorrow Research, and due to the scale disparity between future and tomorrow, in that some destinies are unalterable (thanks to chaos theory).