It is never obvious that the Dark Side wins.
If we’re talking about replicators versus fun theorists, then in this circumstance the rational fun theorist will grow as fast as possible (since it is possible) up until exponential growth hits a barrier, and only then begin devoting any resources to fun. It still loses to the replicator but not by much.
Among humans the Light Side will use generally different tactics and will seek other advantages.
In this particular case, the Light charity is like a bacteria that you’ve engineered to produce a desired protein that you want that is not needed for its own survival. When you put these bacteria in a bioreactor, mutations inevitably take some back to the wild type, which don’t make that protein but put all their energy into reproduction. They quickly take over the bioreactor and drive the “altruistic” bacteria into extinction. This is not a PD case where some equilibrium arises between exploitation and cooperation. Without some countervailing force not specified here, exploitation wins.