Most CBDCs are not going to be using blockchain, so far the ones I know of are not making use of blockchain. The CBDC that we are all talking about are still the same fiat, but in a digital form. They have no connection with Bitcoin, so there is no way that these digital currencies are going to have any effect on Bitcoin.
Like you said, Bitcoin is decentralized and those who wants to make use of decentralized money will continue to make use of Bitcoin, while those that have chosen to make use of fiat will continue with that. It’s a matter of choice, none is stopping the other. Except, maybe Bitcoin is banned in your country. But even if it’s banned in your country, you can still make use of it.
CBDCs are clearly not supporting bitcoin because its purpose definitely in line with fiat but CBDCs are also known to reduce the risks from using digital currencies in their form. But, they are not into promoting bitcoin and cryptocurrencies.
Bitcoin will always chose to be decentralized, while fiat will remain centralized. This is the reason why bitcoin could be a good alternative to fiat so that people who will love to use bitcoin can use it freely while fiat supporters will continue using it. But for those who want to use them both, they will have the freedom to use them for how long they want.
Those coins are nothing more but the response of governments against bitcoin, they are realizing that stopping something like bitcoin is simply impossible, so they are going to do their next move which is to try to deceive people back into fiat currencies once again.
By doing this they are hoping to slow down the adoption of bitcoin by claiming that their coins are better and that they are more secure, unfortunately a great deal of people are going to fall into that trap and are going to believe the governments and they will keep using those coins despite the huge disadvantages that they bring to them, but at the end those coins are going to fail on the purpose of luring people like us to those coins.