It's still right there, it'd take less than 5 minutes to use it, especially considering gods can teleport.
TBH the Rewind Spring feels like an afterthought, it's unnecessary and very detrimental for the story (and any future story). Just having something that reverts things to a prior state makes EVERYTHING less important and kinda pointless, not to mention it's something really hard to work around.
Why was the Rewind Spring the only way to heal Pit's wings? Can't Palutena or Viridi heal him? Or they could just kill him and revive him like Palutena does with all her soldiers.
Why didn't Medusa or Pandora use it? (sooner in the case of Pandora)
What is stopping Hades' spirit from going there and recovering his body? (kinda like Pandora did)
That's not even going retroactively to the first game (which is understandable, the Rewind Spring wasn't even a concept back than).
A possible workaround is making it so the Spring doesn't work on gods, but even that doesn't explain why Pandora waited for so long to use it (and the game didn't say anything about it so it'd be me making stuff up to justify bad writing).