The Celestials are so powerful they may as well be omnipotent from our perspective. It wouldn't surprise me if they can do things like viewing the future or alternative futures. Stuff that Strange needs an Infinity Stone for should be well within Arishem and the other Celestials basic capabilities.

My guess is the Celestials are not many in number like they once were and probably trying to boost those numbers up. In the comics there used to be countless Celestials before they got purged down to very few. But if something is gonna happen like a cosmic scale halfway genocide my guess is they would act unless they perceived everyone would be back in five years which less than a blink of an eye to them.

Also the MCU at that point is still bound up in He Who Remains sacred timeline. Even if there are alternatives measures that could be taken, everything and everyone from the most insignificant human to literal cosmic super deities are bound by his script for the timeline.