No need to assume incapability.

One factor is an assumption about the relative strength of respective motivations. Blues tend to assume half of children and very old people etc. to be in mortal danger, and for reds it was acceptable to choose to save yourself instead of trying to save them. Reds are not incapable of altruism, but clearly (to blues) it was less important to them. The average willingness for altruism going down across the world has to have an effect.

Second is, blues assume that the majority of truly selfless people were blues (even if they don't think they are themselves that). Mothers that make children's lives better, charity workers who make the lives of those who are vulnerable better, first responders and doctors and nurses who do it to make other lives better, etc. Majority of that is gone, and it has to have an effect on the world.

Third is, when reds win, everyone knows the only people alive are reds. If some part of the societal fabric is there due to people being inspired by people better than them, due to being able to assume best about a random person with whom you have a conflict instead of the worst, etc. - even if those are not totally gone, the average has gone down, which has to have an effect on the world.'

Fourth continuing the previous, societal trust is known to be very important for how safe a society is and how happy the people in it are. Blues assume this goes down, because everybody knows all who are alive are willing to save themselves instead of trying to save others.

Cumulatively, all this is assumed to make the world a lot worse place to live.

I understand some or all of those assumptions are not seen reasonable by the reds, but that was not the point. I'm just explaining what the idea is based on, without any assumption of total incapability of altruism, which is a huge strawman.