The airlines will be free to choose if they have 1 or 2 pilots onboard - and this will be reflected in the ticket price.
The cost to the passengers of having 2 pilots on an average flight equates to a pittance.
Technically, there's no reason why it can't be done today. It's just the public perception, and this will change over time - particularly when the tickets are cheaper!
There is a reason. How's a computer going to distinguish between a technical fault with certain variables? Is it going to have decision making abilities? Remember, the first crash caused by a computer making the wrong decision (they are categoric, there is no inbetween) or by failing, will almost certainly be the last. Sure, human error is the still single largest cause of crashes (though I read it has come down greatly recently?), but still the best way for humans and their cargo to be transported (psychologically anyway) is by humans. Then again it would be typical of humans to manufacture our own downfall.