All they had to do, was have Zamasu's 'infinite form' be achieved earlier, where he could just release his own soul across the universe/timelines while his physical body is still around. Make it clear that even if his body is destroyed completely, somehow, without being able to regenerate, it doesn't matter, because his soul's everywhere. Makes him seem insurmountable. And then Trunks' ridiculous Spirit Bomb Sword is what does him in, both body and soul, wiping him away completely. Trunks gets the final, heroic blow to end the villain instead of football-head being called in on speed-dial by Goku to handle their mess.I said years ago that arc shit the bed in the end and I still stand by it
completely botched the landing
Extra bonus, Trunks' original future timeline doesn't get nuked like it does in current canon .