There are worlds in which slower economic growth is good news, and worlds in which it's not.

Indeed. The question of "would X be better" usually is shorthand for "would X be better, all else being equal", and since in this case X is an integrated quantity over basically all human activity it's impossible for all else to be equal. To make the question well defined you have to specify what other influences go into the change in economic growth. Even in the restricted question where we look at various ways that charity and activism might increase economic growth, it looks likely that different charities and different policy changes would have different effects on FAI development.