Well, ad absurdum, as in, in the crazy extreme of what could happen. Halfway would be pretty seismic as well. And some very powerful industries tried to stop p2p file sharing, they couldn't. Any individual more powerful than a whole industry?
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.