If that’s easy or difficult to do from a programming point of view is hard to tell based on our knowledge. Right now, game time and everything that happens in the zoo are clearly closely linked. Time passes faster = it’s not just the animation that’s affected, but all management tasks, guests actions, etc. are sped up as well. Guests spend less game time eating their hamburgers, getting hungry again faster, buying food more frequently, looking at animals, getting upset about something, etc.
You basically cannot separate the animation speed from what’s happening underneath. Slowing down the game speed without slowing down the animation means guests spend months eating a hamburger even if the animation of them eating is long over, months before they feel the urge to move on to see the next animal even if they cover the distance between the old and the new habitat at exactly the same pace as before, months before they decide to donate money - even if the actual donation animation is as fast as before. If guest spawning is not reduced, the zoo will be overcrowded in no time because guests don’t feel the urge to do anything for a long time and just stand around at the same place forever. If guest spawning is reduced, - paired with the much longer time before guests feel the need to spend money - your zoo will most likely go bankrupt in the foreseeable future. You won’t have the funds to even buy new cash animals and expand your zoo.
It’s a very complex system of interdependencies. The developers tried to address the issue by adjusting the animals’ life cycles. This way they avoided the above issues. But obviously there is a limit to how much longer they let animals live.