All these trait could have easily be put into a character that was not resurected. All the best fantasy work have character that evolve based on their life, their hard work and all, but isekai are nearly always just here for the tag.
And this is such a case, author could easily remove the isekai and things would have worked just fine. It would actualy be better as the cliché "my past life was bad" ,that only exist for self insert, would not be present as it add nothing to the story.
Imagine if story like Berzerk or Frieren were isekai, sure the authors could have done it, but the isekai would still be a pointless addition made to just check the isekai tag.
I disagree. Will absolutely needed to be reincarnated for his story to be as poignant as it is - he needs to be young enough for Mary, Blood, and Gus to raise, but he also needs to have lived enough of a life poorly to want completely reinvent himself. That basically means reincarnation, I can't think of too many other options storytellingwise to achieve the same thing. If we take away the reincarnation, it either undermines Blood and Mary's sacrifice because they didn't raise Will, or it leaves the audience wondering why Will is so willing to go balls to the wall no matter whether he's fighting evil gods or dragons.
I feel like the core message of this tale is that you can never, ever give up no matter how dark it is and no matter how many setbacks you experience. And that message hits harder when the main character's starting point isn't a blank slate, but is instead someone who's already experienced hardship and given up because of it. The very first page of the first volume really highlighted this for me. "I made a mistake somewhere, I messed up some part of my life." "I was living like stagnated water." "Being alive is painful, but I didn't have the resolution to die." "I'd be better off dead." But then he sees Gracefeel's lamp and suddenly he gets reborn. Suddenly, he has a
purpose. He's going to spread that light to others and light the way for them, becoming a paladin. Because he
knows what it's like to live without any hope. He knows how bad it is. So he's going to take his second chance and use it for good. And personally, I think that is a big part of why this story is so good. It's isekai that's well written, with depth and nuance like an actual work of fantasy literature.
Will reincarnating from the
modern world is, perhaps, unnecessary, but I feel like he would be less sympathetic to the audience if he was simply some medieval peasant or the like. How many of us have experienced that kind of raw depression in our lives? The feeling that we don't matter, that we'll never amount to anything? That we're trapped in place like a stagnant puddle? I think that's something everyone in the modern world's experienced at one point or another. By making him like us, he becomes more sympathetic than if he was some strange person from an alien culture we don't truly understand from the word go.
Whether Will needed to be the failure NEET he was, I don't know, but I think it's better that way than the alternatives. "FailNEET who fucked up his life and wanted to die gets a second chance to do things better and help others" is a pretty compelling premise, IMO. But you can't easily get that without being an isekai.