If I'm not mistaken, the original manga specifically mentions Zamasu's immortality, although the anime specifically refers to Zamasu, but in the manga we seem to see a mistranslation. However, if you mix the two, an immortal being like Zamasu is considered a "cheat," even though we have Majin Buu and Cell.
So, even if Goku erased Zamasu with Hakai, he should have been able to regenerate his body. Null's response about splitting the timeline is an anime exclusive; I've also provided an album with an explanation not much different from what Null said.
So, in conclusion, regardless of history, the "Zero Mortal Plan" will always exist, even if Future Trunks doesn't return or returns to the present. That's why the same three events repeat themselves; it's like history is already written.
- Goku's Timeline
- Future Trunks' Timeline
- Future Trunks' New Timeline