In an scenario where stablecoins replace CBDCs, I think central banks will become irrelevant in the future. They will cease to exist as governments and stablecoin issuers take complete control of monetary policy.
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.