This sticks with me because Frieren is an elf who lived 1000 years without caring about humans. It was only after Himmel's death that she started to regret not knowing him. So was his death the necessary catalyst, or could she have learned that while he was still alive?
i think it had to be his death. frieren had a thousand years of brushing humans off and himmel alive was just part of that pattern for her, she only felt the loss once he was gone for good. the regret is what made her actually look at fern and stark properly. do you think she would have taken the journey at all if he had lived?