By 2140, there will only be 21 million bitcoins, and no more, making it far more rare than gold. I think $1,500 is toward the bottom end of the scale. Particularly with the world's failing fiat currencies, and the coming hyperinflation, bitcoin should easily exceed $100,000 within a decade, and more. This is assuming, of course, that our fragile electronics survive solar flares, EMT phenomena, attacks, etc. Electronics are bitcoin's Achilles heel
It depends on how you look at Bitcoin's rarity aspect. 1 BTC = 100,000,000 satoshis. By the time the price goes up, services, exchanges, merchants, etc, will most likely completely start using the mBTC or Bits unit. If you completely switch to mBTC, and especially Bits, then Bitcoin's rarity aspect isn't that much of a strong argument anymore. Only thing left to say is that the supply of coins is fixed at a certain number of satoshis, mBTC, Bits, etc.
I can not understand where such forecasts come from with such huge sums. Really really Bitcoin after some years will cost an order of hundred thousand dollars.