Your argument relies on the postulate that Bitcoin is going to become a global standard. There isn't anything coercing the world economy into adopting Bitcoin. On the other hand, "those with a big stash of the old currency" already have enough wealth that they could prevent Bitcoin's widespread adoption.
yes there is, you can transfer x $billion to whoever you want with BTC, across borders and jurisdictions, in about 10 minutes, for free, with a 10T market cap this will be done.
No other financial system gives this sort of power and flexibility, to every one/entity. Imagine the amount of banking infrastructure, laws, regulation, and all the associated cost in running it.
There's probably at least 2 trillion tied up in realestate, wages, IT, regulation, that can go from the retail and commercial banking sector alone due to bitcoin, and ok it takes money to run BTC, but that's where PPC or Emunie come in, to meet that issue, cheap to run as well.