And, who told you that exchange-based tokens or coins are decentralized? They are not completely decentralized despite the fact that they run within blockchain networks and the transactions aren't really controlled by the parent exchanges but they can easily be modified or manipulated.
I am aware exchange coins are not as decentralized as they claim to be. Especially BNB which is heavily-manipulated by Binance. You can't say that CBDCs "won't operate on a Blockchain network", especially when governments and central banks have expressed their interest towards the tech. They could launch a CBDC that lives within a private blockchain network controlled by one or more banking institutions. Don't expect them to use a public blockchain network like Bitcoin, Ethereum, or XRP, though.
We're going to have to see whenever governments will allow stablecoins to co-exist with CBDCs or not. It's easy to take down a centralized stablecoin than it's the case with a decentralized cryptocurrency like Bitcoin. The future is widely unpredictable, so we can only hope for the best.