If half of us care only about ourselves and those closest to us, and only a smaller, committed fraction of altruists are willing and capable of taking action to benefit the rest of the world (including future generations), we may not achieve an “ideal world” (as I, a somewhat committed altruist, define it); but historical evidence clearly shows that this has not kept the world from getting better.
Those on the other side of the spectrum rightly point to economic growth (the capitalist form being based on the premise that everyone is selfish) as a key driver in improving the material aspect of quality of life for an exponentially increasing number of people over the past century. Since our knowledge of impact on others and the future can be imperfect at best, they argue that we are practically limited to taking care of those around us, and letting the rest of the world take care of itself. They point to socialist countries like the former USSR as clear examples that centralized systems, ostensibly designed around the altruistic theory of communism (in its simplest form: taking from the rich and giving to the poor), do not and can not work efficiently.
For most of my life I was a staunch Republican. I agreed with my father and his cohorts that truly helping people involves enabling them to become self-reliant rather than simply giving them what they need or want. The apparent Robin Hood mentality of the “do-gooder” Democrats looked like a recipe for disaster: bringing down all of society to spoil the weakest of us all. I didn’t understand at the time (due to lack of questioning) that Democrats actually, on average, agreed that people should become self-reliant, but they felt no one should be deprived of basic needs, a philosophy more closely aligned with basic Christian teaching than that of Republicans who acted more like the ancient Pharisees.
It should be said that I now believe that my father was in fact an altruist (he simply couldn’t accept that side of himself), but he chose to focus his altruism on helping people take care of themselves, primarily through education, rather than advocating the distribution of wealth to them. I have personally chosen to take the extra steps of supporting (to the extent possible) the distribution of enough wealth to keep everyone in the population above a basic survival threshold, and promoting responsibility for improving the chances of survival for future generations.