Customers of the cartel naturally transact at the cartel's website or retail POS terminals. So the cartel can control these transactions and starve the Bitcoin network of these revenues.
This supposes that hardware wallets will be offline and not communicating with the public network that far in the future. Every wallet should have this feature, such that two individual parties (that do not trust each other) could transfer coin between two wallets (otherwise it's a race to see who can get online first to double-spend or claim).