It's often said that people who don't want children are selfish, but what altruistic reasons for having children exist?

people are mostly fond of using them as guinea pigs and they are mostly called parents

Interesting question. In modern times (and affluent societies) children are often created for sentimental reasons or it might also involve the need to continue you blood (on a primitive level). So yes, not having children (out of choice) might not be so selfish after all.

🤔 I can think of some altruistic reasons for not having kids..

I supposse they are some people would simply say that reproduction at it's most basic...means the survival of the human species, suppossedly an instinct in all beings. So it could be seen as altruistic in that sense. Of course some child may grow up and blow everyone to bits...so I guess it's all simply a big gamble

Children should be the ultimate result of love between the parents.
The ideal of their hopes and dreams, if they manage to raise them right.

People like that need to mind their own business. Deciding not to have children doesn't affect them. They also forget to remember people breed for selfish reasons and rather breed than adopt.