Altruism is not a myth; behavoral psychologists may wish you to think it is, as might select oppressive political theorists. Sophists might argue for the non-existence of virtually any sort of characteristic as being unreal - that doesn't make it so.
Human behavior and motivations for behavior from thought are something less than manifest material objects. Materialists seeks to make everything that is real material, while thought and motivation have somewhat more of a metaphysical aspect, that is the reasons for thinking are not easily related to a materially determinate causal relation. Altruism as a concept and any motivational line of reasoning might radily be argued not to exist - for they are not material objects.
The difference between material and immaterial arguments are significant. Thought is supposed by many to be free; altruism is an example of free thought determining to confer some benefit upon another without receiving any significant benefit for doing so. Sophists might argue that the pleasure gained by giving without expecting anything in return is itself a negator of altruism; the ability to relate any metaphysical argument to another without logical boundaries of definition is perhaps simpler than it is with some material objects...although that is not certain either.
Human language is a use functional means of communicating ideas; it is not as precise as mathematics in many respects. Language is a structured approximation of meanings in referent terminology of words to objects intrasocially. Many of the arguments that people make such that A is proposed to be a myth are a function of substituting meanings of terms such as A has in one context use and in another.
Consider if I say that action A is B, and someone arguing with that says that action A is not B; for the argument to have any meaning each must agree upon the meaning of B. If altruism in ordinary usage means performing action A, then B exists whenever A occurs...it is as simple as that. One may change the meaning of the term B such that it does not occur when A does, and language may do that sometimes (evolve meanings of words). It would be unhelpful to say that individuals do not do things that primarily benefit others more than themselves, and that the meaning of altruism should be erased as a valid concept placing it in to mythical status. The premise that people in actuality haven't sufficient free will to not help others altruistically-that they are compelled to help others is silly.
One may have a completely deterministic world view such that altruism and all other concepts of expressions free will are illusory, I believe that is invalid. One may find a general, pervasive reason why individuals that perform altruistic actions get something back equal to what they give necessarily, yet most people would disagree with that. The problem is that most actions of altruism involve conferring some material benefit, intellectual or material capital, investment of personal time or sacrifice of health and so forth to others without commensurate or significant reciprocal exchange. The exchange theory of value that functions in the marketplace is at least plainly able to prove that altruism from the traditional standpoint definitely exists. Circumstances where in altruism actually occurs may be incorrectly recognized, for in some circumstances such as loud public giving by celebrities or mafiosa politicians the return in good p.r. may substantially compensate the financial value of what was given 'altruistically' .
Stoic philosophers at least believed that virtue is the only good, and perhaps giving in such a context as an effort to secure the good for oneself through virtuous actions overturns altruist actions in favor of the self-ish effort to be virtuous. If virtue sometimes requires the sacrifice of being altruistic does not that fulfill the point of altruism rather than to degrade it? If virtue is in doing what is right regardless of the personal cost, so far as it is measured and right to do so in that it will accomplish the virtuous, it would be a contradictory explanation to claim that when virtue (altruism) is accomplished it is no longer virtue.