If instead of losing a male lion due to low male slots, what if the game asked "would you like to go into dept or lose the lion?" You would lose 600sb and wouldn't be able to buy anything until you got to 0sb (rather than however negative sb), then you continue normally.
This idea may seem stupid, but it is better for those who don't get to play as often. I myself lost a lovely and expensive male because I was sick and asleep for the majority of the day. Others may not have good internet every day, or perhaps something unexpected happens and they can't get to their devices.

I don't know about you, but I'd be happier and I'd stress less about stuff like that. Being able to not play for a few days encourages a healthier lifestyle I guess, since you aren't constantly playing. I don't know, I tried.