will msci world ever reach new ath?

I often hear that you should dca here and there because in the long term (multiple years) it will be way higher than now.

Where do you take this certainty from? Only because of history?

Green politics are more and more bringing economy to slow down due to climatic changes. Why do you think it wont decline slowly but permanently and dont be way higher than now in lets say 5 to 10 years?

Im especially referring to msci world and ftse all world, as those are the ones that are recommended as set and forget.