If that's the only island they play on, then their loss would have no effect (positive or negative) on the player density of the other islands. Overall it would be a net increase in density.
True. That said, I still worry that we'd also lose some people who play on multiple islands but consider one of them their favorite. This sort of hypothesis is pretty much impossible to prove though without surveying the player population, and I suspect that if we did a certain percentage of people would tell the survey that they would quit even if they ultimately didn't intend to in an effort to discourage the Devs from closing their particular favorite island.
