How can a decentralised exchange put US$ in my bank account?
The individual broker you're dealing with would transfer the cash in any of the thousands of ways fiat is transferred now.
There will also evolve intermediaries who's soul business would be to facilitate the fiat transfer in in the system through whatever network they have access to for whatever country they are in.
So finance intermediary plugins could be installed in your P2P exchange app? Might work.