I understand the attachment problem, but that's part and parcel of bringing new animals home. I adopted my pit mix knowing that in 1-2 years I could have to give him up for dog aggression. I'm not totally out of the weeds, yet, although he is 18 months with no sign of dog aggression. Nevertheless, I knew every day and still know that he could come into his temperament and go after one of my older dogs.
I just had to have a talk with my partner about our pit puppy, about how she could develop DA and that would be the end of that. About how if she bites a human with no warning she needs to be put down (as it points to unfixable problems due to poor breeding).
If my pits got into a fight and I couldn't separate them with a break stick, I wouldn't hesitate about which one to save. I have an obligation and made a commitment to the first members of our family, to ensure their quality of life, safety, and health.
Don't take this to mean I don't really love them. I am insanely attached to my young pit mix. He is capable of feats my older dog would never be able to do. I have spent every day since I brought him home working with him for hours on tricks and behavior modification. He is my magnum opus of dogs at the moment.
I am not unsympathetic at all. It's a shitty situation, but it's a possibility I take solemnly into account when bringing new animals into the menagerie.
I will say I've never worried about this among cats. My mother-in-law has five cats, and they achieved their own pecking order. They lay a quick smack down on one another regularly, no serious fights though.
I got my kittens (who are now my mother's cats) from a farm that had 40 cats living there due to being dropped off (abandoned) unfixed etc. on the farm. They had a very sophisticated hierarchy, and I never saw a true cat fight (torn off ears, bloodied eyes, dead cats) break out. Just a "No, I'm the boss in this situation" spat every now and then between older cats and younger upstarts. 39 cats would wait while a 15-year-old cat named Princess ate first. Sometimes an adolescent kitten would try to approach the food, and Princess would give a quick right hook and go back to eating her fill. When she was done, the cats came in waves, like there were pre-defined platoons of cats whose turn it was. Really amazing to watch. They ate a 30# bag of food every day out of a horse trough.