Ojisan already lived a long fulfilling life and died because of old age in the isekai. He was reincarnated in modern Tokyo because the spirits recalled Ojisan couldn't stop talking about games and in a whim, wanted to see modern Tokyo.
The story will end with Takafumi being closer to Fujimiya and Ojisan dies, consciousness fading away while surrounded by his isekai friends. He dies because he's already dead when he's 17. Takafumi will realize this is Ojisan's fate since there's an ending to Ojisan's isekai adventures in the notebook and correctly guessed why the spirits revived Ojisan.
Then in the omake, the story will be narrated by Takafumi, trying to decide if Ojisan did revive in modern Tokyo instead of the entire story being his own fantasy since chapter 1. In the end, Takafumi decided that it doesn't matter. But the truth is Ojisan used his memory wipe spell to erase his revival in modern Tokyo from everyone else except Takafumi. We see Takafumi living among the modern Tokyo cast with corrupted personalities because of the memory corruption wipe spell