Digital dollar is the same as fiat- the only difference is the platform on where you can use it. In contrast, BTC is something that can be used as alternative payment if you want to secure your transactions in the blockchain. Again, like what I similarly comment on posts like these, both fiat/digital dollar and bitcoin can co-exist with each other. They can both be used as they can fulfill each other's roles effectively.
The Digital Dollar could become a threat to Bitcoin, if the US government forces its use. But that's very unlikely to happen, especially when the US is a democracy. What will happen is that the government will tighten regulations to keep away the "bad actors" from Bitcoin. With how far the cryptocurrency has gone since its inception, it's very unlikely it'll disappear just like that.
I think both Bitcoin and the US Digital Dollar (or any other CBDC for that matter) will live alongside each other for generations. Sort of how Gold is living alongside Fiat up to this date. People will choose which kind of currency to use for day-to-day payments. Who knows if we're closer to a cashless society than we've thought? Just my thoughts