If it is a big centralized exchange，that should be relatively safe，but it's certainly not foolproof. And the important is，we all know decentralized exchange is better，but the reality is the trading volunme of centralized exchange is much larger than decentralized exchange‘s. So i think that existed is rational.
Yes, obviously very rational, because between centralized exchanges and decentralized exchanges, each of them provides a good service for their users and everyone will know which one they will need according to what they need.