Visa is a tiny little middleman compared to a Federal Reserve, however crazy that may sound. Visa is a) only one of similar middleman, b) it has no power over bank accounts or any other entity holding USD, it's only involved during a transaction. In fact, I could probably survive without touching Visa/MC for a long time (and without touching their Chinese or other alternatives). I can for example wire money from bank acc to bank acc directly and people can self organize to do that at scale and not involve a proxy. It's just won't be as convenient, no cards, no card terminals etc. There are multiple proto alternatives too, for example BLIK in Poland. EU has pretty robust bank system.
All that text above is just underscore this - a future Fed owned token would be on a completely different level of control. Cross border control too.
Regarding alternative token systems - a) outside of maybe 2-5 tokens all others won't be adopted and die out (unless enforced in a totalitarian way), b) the survivors will share the same traits, copied from one to another, after all they will be controlled by the same orgs - central banks.
And finally - no, you won't be able to sell tokens to money. Only if the owner of all tokens (a central bank) will graciously allow it. For example in the pilot in my country it did not. Thinking logically - if token system is created to restrict freedoms and bring more control to the govt, why would that govt allow "exiting" this new system? Everyone who cares and all potential targets will sell this token and go back to using money. So of course any bridges will be either heavily restricted (for the inner circle, corrupt officials, oligarchs etc.) or won't be allowed at all from the start.