I'm not sure I understand the economics here. I'm concerned about the final stage which is fee-based. I'm not sure how this fee would get set. Would the fee structure mimic the profitabity of mining for bitcoins? If not, would that drive out a lot of the network perhaps to the detriment of security?
Miners can collectively demand a fee as high as people are willing to pay. And mining will become cheaper with future technology, and will also become cheaper if a lot of people stop mining. There's not much we can do now, though. The block reward can't continue forever, so the only reward will be the fees.