As long as Bitcoin will be a viable currency (which, as things look now, seems to be a REALLY long time) mining will exist.
And there's the nub of it. Either bitcoin is:
a) A viable currency that will continue forever more.
b) Something that seems to be giving us free money.
Or to put it even more simply - are you mining coins to sell them for cash, or mining them to buy goods/services?
Now there are always going to be those people who will mine and sell them - but these people are only supported by people buying newly minted coins for cash.
So what motivates these purchasers? Either they want to buy/sell with them - or they're just hoping the price goes up and they can sell for a profit.
I dunno how this will all end up in the short term, but I just have a sneaky suspicion that 90% of people involved in this are looking at their bitcoins in dollars.
Say you are mining 2 coins a day. Say you can't just swap them for cash - how are you going to spend your bitcoins?