Yeah, many countries are trying to launch their own cbdcs, But its never and ever overtake bitcoin. because bitcoin will be accept in any exchange.
There is no need for those projects/CBDCs to take over bitcoin based on its usage. One thing that they only need to do is to be used by their citizens and have them used for so many ways. 1. Forcing them to use.
2. Educating them to raise awareness of the existence of their CBDC.
3. There's no need for them to destroy bitcoin's reputation just for their citizens to use it.
But i have one doubt brother, " can anyone start a crypto exchange without bitcoin ? 
DEXes, they're mainly focused in altcoins.