If T-Rex is the big prize it should be in the higher arenas. New players don’t really need T-Rex DNA, Ankylosaurus is perfecly useful for them. And Rex is after all a global spawn. Incubators are supposed to help you out a bit with the DNA, not be your main DNA income. New players can hunt Rex out in the wild. However having Rex in Arena 1 was making many high level players downgrade on purpose to get it, which meant a lot more unfairness for newbies.
Rex isn’t that good anyway. She is useful for making hybrids but she needs a health buff to become good for higher arenas. As it is now, she dies way too fast. She is one of my weakest team members, sometimes I think I keep her in my team just because I’m fond of her.