That was some [i]terrible[/i] immortality, Mokou had to reflect; she was certain that if Eirin was to see it should be wincing at how badly applied it was. It looked kind of like someone had heard about the Hourai Elixir, but forgotten that it stopped you from ageing. Or tried to make it without Kaguya around, leading to a potion that stopped you from dying but absolutely nothing else. Thinking about the Hourai Elixir probably explained her next words, too.

"Yeah, sure; if dying is the worst that can happen then it's got to be more fun than spending a few hundred more years in a forest. It's not like I can die."

He could keep the wish, though. The only thing she could possibly ask for was Kaguya's death--and if the wishes were made by changing possibilities, then he couldn't make [i]that[/i] wish come true. Changing possibilities couldn't make the impossible come about.