If a population really did adopt a single payment network, the goverment could issue a blacklist of accounts that you can be punished for transacting with.

think of the power the US government has to freeze the funds of russian oligarchs - blockchain allows at least a version of this: not to freeze the funds, but to prohibit sending or receiving funds from a certain address

vendors could happily subscribe to this blacklist to ensure legal compliance (an RSS feed will do but thats just me)