By selfishness some people just mean acting in ways that further your interests. But if that is what is meant by selfishness then the word is useless, as by that definition everyone is selfish. Even a man who gives all he has to others is selfish, because he gave up his goods because he wanted to. And doing what he wants to do is acting to further his interests, and that would be selfish. So when I use the word selfish I don’t mean someone who acts to further their own interests, because everyone does that, but someone whose interests all directly benefit their own wellbeing (by giving them more power or opportunities). Thus someone who gives everything they own to others is not selfish, because even though they are satisfied with actions they are made less powerful and have fewer opportunities as a result.
Selfishness may or may not be practically self-defeating. Certainly a community of selfish individuals defeats their own purpose, because they all end up worse off than a similar community that isn’t dominated by selfishness (due to prisoner’s dilemma type situations). And usually successful communities will do their best to punish and exclude selfish individuals, meaning again that the selfish individual may end up worse off by being selfish. So to be a successful selfish individual involves living in a community of predominantly selfless people and pretending to be selfless yourself. Of course doing that implies living in a way usually contradictory to your fundamental desires, which I think is probably a good reason to give up on selfishness in its own right.
But let’s assume that such a selfish person can thrive, even if they have to pretend to be selfless most of the time. Even so there is another argument we can bring to bear against selfishness, although admittedly it only is relevant to a selfish individual of sufficient intelligence. A selfish person who focuses solely on what is in front of them, and who never stops to think about why they make the choices that they do is completely unaffected. But suppose they do. Then the selfish person will surely say that they are trying to maximize their overall wellbeing, both in the present and in the future. But in the long term everyone is dead, selfish and unselfish alike. Given that the selfish individual will die, and that there is little they can do to prevent it, what is the point of being selfish?
To take another perspective on the issue, we observe that once we are dead our wellbeing cannot be affected by any actions. And the time when we are dead dwarfs by far the time when we are alive. Thus acting simply to increase our wellbeing seems futile, and from a perspective that includes all time our actions, no matter what they are, seem to have only a negligible effect on our wellbeing. Why bother when in the long run it all averages out to the same anyways? Such lines of thought can lead to a kind of nihilism, in which the selfish person is led to the conclusion that their selfishness itself doesn’t matter, and that there is no point to their existence.
Of course the purely selfish person is free to try to ignore these considerations, to focus so intently on the here and now that such extremely long term considerations are put out of mind. Which is in effect to abandon a rational perspective for something more comforting. This is of course possible, but it is certainly harder to do the more intelligent the selfish person is, because generally an intelligent person values their rationality highly (that is usually what makes them intelligent), and hence will have a hard time pushing it away for the sake of convenience. Thus we conclude that this is a good reason not to be completely selfish, in order to avoid this particular kind of nihilism.
However, the selfish person might retort that this same line of reasoning can be brought to bear against everyone, not just the selfish. After all, no matter where your interests lie, everything eventually must be destroyed. Admittedly this might equally lead some unselfish people to nihilism, for example someone who devotes all their energy to protecting someone else. But there are plenty of unselfish people who aren’t drawn by this line of reasoning into some kind of nihilism. One such unaffected group are those whose interest lie in achievements and not in what comes afterwards. For example, someone who is trying to shed some light on a mystery only cares about whether they can solve it or not, not about whether that accomplishment endures. Of course there is also a selfish version of such people, namely those who live only in the moment, with no thought of the future. But such a lifestyle is unwise for other reasons, and so needs not be discussed. Secondly, we must consider that whether everything will be destroyed eventually is not exactly set in stone. For example, someone who was devoted to the wellbeing of Germany did not have their efforts undone just because Germany was divided for a time; their interests were directed at the German people, not a specific political entity. And it is unlikely that the German people will disappear anytime soon. Indeed there are many things we might expect to endure for a very long time, although not only under the same name, which we might devote ourselves to some of the time. If such things are ever to be destroyed it may very well be because of the end of the universe or some completely unexpected event (like a meteor impact). But such ends are simply speculation. The end of the universe is so far in the future that there may very well be some way to avoid that fate discovered between now and then (certainly we have more than enough time). So with regard to such long lasting entities their destruction is not certain, and is certainly far less certain than the destruction of particular individuals. And so with respect to these considerations many unselfish people find themselves in a much better position, and are not necessarily led to some kind of nihilism like the selfish person is.