There are other means of passing information between individuals in a population, e.g. by communication, upbringing, education, propaganda.
E.g.
Step 2. Extremely altruistic people cease reproduction and use the time and energy they would otherwise have spent on their own children to educate and encourage others to become more altruistic.
... and you wind up with those more predisposed to being altruistic, upon education and encouragement, also foregoing reproduction. The result may be a short term surge in altruistic behavior, but a stronger selection away from altruism at the genetic level.
E.g.
Step 2. Extremely altruistic people cease reproduction and use the time and energy they would otherwise have spent on their own children to educate and encourage others to become more altruistic.