I can't 100% fault the guy without hearing the story from his side, but obviously from Tierra's perspective she got forgotten too easily, especially if you consider that for her as a long lived race one year might feel like a day.
This might complicate things with Demonio considering that he also won't live as long as her and he's building an harem of milfs. But there is no doubt that the protagonist will be able to show the sincerity of his feelings as the greatest mama lover ever.
Elf mama's issue with her previous lover wasn't that he died or his short lifespan, it was sorely because of how fast he moved on.
Even in human lifespan, 1 year of moving on after promising to wait for someone is insulting. It's like your partner going abroad for maybe a decade and promising to wait for each other, then when your partner comes back you already married someone else after just 1 year, with kids to boot during that 10 years absence.
Demonio, on his part promised that he'll fix the problem with the world tree and won't let her be the sacrifice again. For Elf mama, the promise part is the most important part.