Thats a lot of varying factors to speculate on
. Shame we wont be around to see how it all pans out.
Fortunately the process is self regulating.
If there isn't enough profit for miners, then they will stop mining (either due to intelligent financial decision making, or because they can't afford to pay their electric bill any longer). As there are less miners in the system, there are less miners to have to split the block reward (transaction fees) with in the pool. This increases the total bitcoins earned by the remaining miners (who have access to efficient enough equipment, or cheap enough electricity to maintain profitability).
Where it could potentially run into a problem if the transaction fees per kilobyte are too small and the maximum blocksize is too small, is that there may not be enough miners remaining in the system to offer adequate protection from a 51% attack.
On the other hand, if bitcoin has extremely wide acceptance and usage, then very large businesses may find that it makes financial sense to contribute a small percentage of their revenue to mining at a slight loss to protect the payment system that they've adopted.
