google states 70 trillion is earned by the world in a year, so 5 trillion is not that impossible.
I think ethereum will do really well these coming years over bitcoin though, it has more reason to become valuable in my opinion.
Also i did watch a video where it says that once bitcoin has mined all its coin it might become worthless which is in about 50 years, but they do have some options round it its just if people take to it.
RE: The Winklevoss twins: BTC future market cap at over $5 trln