I don't really understand your logic here. CBDCs and stablecoins are a product for retail ("private" users and non-banking businesses), while the Central Bank would still have full control over their relationship with banks via the fractional reserve system and Central bank interest rates, which are the main instruments for monetary policy today. That wouldn't change if CDBC plans are scrapped in favour of a cooperation with stablecoin issuers.
If banks disappeared, then your prediction could become true indeed. But banks' main purpose is the generation of credit, i.e. the management of loans, which involves evaluating different risks all the time and decide how much you can lend to a certain person or company based on their risk profile. That service will have demand forever (at least in market economies).
Yes, stablecoin companies could do that too, but they wouldn't have a competitive advantage over a bank which has a lot of experience in that sector. And "traditional" banks could also become stablecoin companies themselves, or use stablecoins from other companies for that purpose.
The generation of credit, would be handled by stablecoin companies themselves. In essence, they would become banks' replacement. To put it simply for you, only two entities will be in-charge of the new digital Fiat money system (powered by stablecoins): governments and corporations (aka stablecoin issuers). Fees, and everything else would be charged and handled by a centralized app under such entities' control. You would be able to deposit, withdraw, and check your balance. But you won't be able to have access to your keys/seeds (not your keys, not your coins). Sort of like how a credit card works.
Since stablecoin issuers have the authority to blacklist/freeze accounts, stablecoins would effectively become CBDCs (but without central banks). Monetary policy would be handled directly by the government itself. Retail banks and central banks would be out of the picture. At least, that's what I think will happen in the future. Only time will tell...