Allowing for that rational line being above zero, the question is if that rational limit pays for the security we need to sustain. See my previous calc. on transaction fees needed.
https://gist.github.com/gavinandresen/5044482I am not sure the numbers are completely sound so I am not saying rely on them like gospel but more as a thought exercise. I think the underlying study that Gavin relied on has some pretty worst case assumptions backed in and the average miner is probably going to be better connected than the average non-miner. Still even assuming they estimate is 10x actual cost the idea that there is no cost is simply not supported. A 1 satoshi fee (or even 100 satoshi fee) for all intents and purposes is a no fee transaction.
As for fees making up the difference of the subsidy cut ... they won't. However at the current time the network is probably overprotected relative to the actual economic value of the transactions occurring on it. Subsidies tend to do that in any market. So over the next five years the difference caused by the two halvings will be compensated by a combination of a) some reduction in overall security, b)the rise in the exchange rate as miners costs are mostly in fiat terms, and c)rise in overall block fees.