I would love to see the cart make a come back. I grew up on the NES so I have a soft spot for carts in my heart. But i would not want to buy Fallout 3 on a cart at this time. I don't think that I have 'that' much money.
If it would be affordable, then yes they should return. Money is one of the main reasons why discs have become the norm now. I don't see that changing anytime soon.