MC is over a billion years old, but still has the same wimpy personality as when he was a teenager as though he hasn't aged a day. You'd think he would've changed a little after living in solitude for such an impossibly long time. There's no way he would still be perfectly sane after all that time, let alone unchanged. At least he should be completely apathetic since he's so old and nothing happening in the story matters at all when viewed from that scale. The fact that MC isn't ridiculously OP after all that is also weird and sad. Seriously, did he spend 99.999999% of that time in a coma?
The story could've been about an overpowered MC who's lost his humanity being thrust back into an old life he's forgotten and dealing with things (humans) that he doesn't remember what they are. Deciding if humans are things that should be killed or not, learning how to communicate, regaining some semblance of emotion, figuring out how the real world works, slowly recovering old memories, etc. You know, interesting things. Sure the story would've gone in a completely different direction, but we didn't really need another "MC goes to fantasy school" story, especially since the story we got makes no sense in context.
Instead of getting something interesting, we got something generic. Oh well.
