Bhagavad Gita, Chapter 6, Verse 43

There he is united with the intelligence acquired in his former body, and strives more than before, for perfection, O son of the Kurus.

Nothing actually has to start from zero, real growth carries forward, even across a gap that looks like a fresh start.

This directly answers Arjuna's original fear. Far from losing everything, someone in this position picks back up with real momentum already built in, the previous effort wasn't wasted, it was simply paused.