I don't think the existence or non-existence of Onan is important: he surely does not exist in his former shape or form now. He was symbolic of the need for a man to care for his brother's widow, and for the tribe to survive. There was no Social Security in Hebraic times, and lots of tribes have ceased to be because they did not have the tenacity of the Jews. No one still considers themselves to be a Visigoth, an Alan, a Pottawattomie, or a Gaul these days.
It's not that these people have not procreated; but as a tribe with their customs they have ceased to be.
Without major medical attention, a premature fetus has little or no chance of becoming a human being.
I am not even sure that all births surviving is not an inherently good thing for the planet or the rest of humanity. There are too many people to sustainably survive now, probably, and eventually the birth rate will result in the collapse of the population due to excess numbers.
I continue to consider the decision of an individual to reproduce as his/her own. Onan had a right to not diddle his brother's wife, and any woman that finds herself pregnant has the inherent right to not give birth. It is none of my business.