Its really hard to be able to look into the future with a crystal ball. But, at least with bitcoins, it seems like we have a clearer picture than most "economic" phenomenon
At the end of the day, the price per bitcoin is going to end up pairing with how difficult it is to get them, and how much people want them.
Its simple supply and demand.
So right now, as we speak, there's MORE supply than there will be later. That's a fact.
Assuming the demand stays the same, that alone would increase the price.
The tightening of supply increase, with stable demand = bitcoins +
What will happen is those that give up mining will sell their rigs
Some will sell their rigs for bitcoins + demand
Some will sell their rigs for cash. X amount of those people will convert some of that money into bitcoins to
Continue as part of the community + demand
to simply speculate prices and still try to make money +demand
What number is that X? I don't know. But its more than ZERO. And can only be beneficial for the bitcoin.
One thing you have to keep in mind -- scarcity does NOT make something valuable in dollars. People have to WANT THEM as well.
If scarcity were the only factor, then my fingernails would be worth $200/ounce. After all, there's only one of me, and I can only produce so many fractions of an ounce of fingernail per week!
So you see, there's more to it than just difficulty. I know, Bitcoins are cool, but there are still *major* obstacles to widespread adoption.
A smart poster on here brought it up a while ago -- he called it the "parents test". Here's how it works: You tell your parents about Bitcoin. Then you leave them with the request, "Now buy some Bitcoin". NOT do it for them. NOT stand over their shoulder as they buy some. Just tell them to buy some! Report back here how it goes. Hopefully I'll be pleasantly surprised.
But think about it: Many of us clever PC techs, programmers, etc. had to read up on how Bitcoin works for a day or two before we (mostly) understood it. Do you think an old Baby Boomer who types e-mails in ALL CAPS is going to want to buy and use Bitcoin?
Sadly, I doubt it.