The answer to this, from what I've read, is that the network will set their own limits at that point. Some miners will still take free transactions, while some will set to only take transactions with X, Y or Z fee, and just depending on how much your fee is depends on how many miners will include that transaction in their block, thus lowering the possible time your transaction would be unconfirmed. This is as I understand it, which might be incorrect.
The problem is that we no longer have a competition of miners, but a competition of pools, lone miners will have no noticeable effect on transaction selection for the foreseeable future. The limited number of transactions pickers in the current mining eco-system, combined with the current complete lack of scalability of transaction verification means that we will see a strong incentive towards fee laden transactions as the market matures. This creates the very real possibility of the mining pools arbitrarily setting rules that bypass the protocol to impose higher fees on the market.
At current we have a 1MB block size limit which is a kludge to prevent transaction spam, even if that limit is not lifted, and the average fee was 0.005BTC per transaction for full blocks a miner would still make 25BTC per block, which is enough to allow miner market equilibrium. This has the twin technical problems that there may not be enough transactions, or they can't be verified fast enough to fill the block. The first problem will fix itself if the market takes off, the second is far from intractable.