Short answer is, yes, it's possible. But it could also be $0/btc in 20yrs. That's part of why this is all so interesting. That article uses some pretty simple-minded logic to derive $/BTC in the future based on the assumption that bitcoin actually becomes a major currency in everyday use for a lot of people. I think it's somewhat difficult to make a case for that realistically happening in 20yrs, given geopolitical dynamics, but who knows.

To me, the easier case to make is simply that bitcoin will increasingly be seen as a better substitute for gold, and people who like to hold gold will also start to hold bitcoin, and will eventually start to hold bitcoin instead of gold (this is discussed here: https://bitcointalk.org/index.php?topic=100065.0 ). If 1% of gold were swapped for BTC, that yields about $4000/BTC. If we're gonna get uber-speculative, I think that's one of the easier cases to make that supports bitcoin becoming super valuable.