I fully agree that it is a horrendously difficult problem. But from cross breeding, most of the time the offspring is sterile. Like a mule or liger. But you're right, things often don't go according to plan.

But there is an overwhelming amount of scientists, who are smarter than me and actually study this, that believe it is worth the risk. I'm willing to put my trust into their judgement. Btw, there has been a LOT of testing. But what I mentioned in the OP was more about that at a certain point you have to take a leap of faith.