@ninjadork Isekai used to be a narrative scenario, but nowadays is a genre that is practically exclusively tied with the Chosen One trope, and has devolved into stories for wish fulfillment.
This particular story would remain the same without the isekai elements, it could have been a typical fantasy story, and the isekai aspect seems to be there just to fit in within the mainstream.
You could have the previous hero team adopt a baby and raise it, explaining why the kid didn't knew about the world, and why he hand't been exposed to its realities like everyone else, then have him encounter the gods and be put in the middle of deciding if he chooses to keep the legacy of his adopted family or join the dark side. It would flow just the same.
The MC's motivations of his previous life don't really have much weight on this story because they can still happen without him being an otherworlder, they could have made him an infant around 5 to 7 years old, young enough to be ignorant of the world at large but conisant enougth to have experienced the lost of his previous family and grew attached to a new one.