That's the problem: IF.
The average citizen of any country almost never uses international payments, so for them bitcoins have no use at all.
Inland payments are for free, in Europe international payments within the IBAN zone are also completely free. So the potential market is maybe not so big at all, especially not in Europe.
Most people who have bitcoins have it purely for speculation, they will never do any international payment.
What if banks would lower their rates to compete with bitcoin when needed?
Sure, the banks can lower their rates but knowing the big and slow banks, they will wake up 10 years too late when they've missed the point. Just how it happened with payday/consumer loans. They're now attempting to get into that market but no-one notices.
I just made payments within the same country in the EU (Swedish bank), I was charged something like 20 cents, while insignificant, it's not free. SEPA payments are €1 I believe.
Europeans still deal with non-Europeans on a regular basis. Small-time work outsourcing is one the rise and most of the workforce is in places like India where banking is still consumer unfriendly and primitive.
Besides, the world is quite a lot bigger.
In the Philippines there's something like 10 million people abroad sending money back home at least bimonthly. They use Western Union and the likes, which are expensive. Most have state of the art phones and often move money to areas that are somewhat remote.
While most people don't need international transfers, a significant portion of the population does. 10% of 7 billion is still 700 million, that's not nothing.