“Having blood types” is a medical frame of reference. The ABO+/- systems is code for the presence or absence of three particular common antigens on our blood cells. They’re important because of the way our immune systems work. There are all sorts of other antigens that our immune system can respond to, to protect us from infections.
I suppose in theory every human could evolve to have the same sets of blood antigens so we could all transfuse blood. ... In order for that to happen though, we’d pretty much need to hook ourselves all up to a giant infusion system, and/or just massacre every human who doesn’t have AB+ blood type. That’s how evolution works.
The advantage in not slaughtering every human without AB+ blood type, is in all the other qualities of those people other than their blood types and the inconvenience of having to organize blood banks.
I don’t know about you, but I figure it’s probably a favorable trade off, to let them live..
On the other hand, reducing the human population could help slow down our industries’ devastation of our planet… but the wars that would break out, out of trying to organize mass slaughter based on blood type, would probably involve even more damage. So let’s not suggest it.