If Satoshi thought that transaction fees would be how mining supported itself, that was just a guess. He couldn't possibly have known how it would eventually work out or
even thought he could know. Note that some of his other guesses have proved wrong, like the popularity of sending directly to IP addresses.
Not quite. Once the initial block reward is exhausted (negligible), fees are the only mining income. So that leaves only two possibilities: transaction fees would be how mining supported itself, or
external incentives cause miners to mine at a loss (fee-wise).
As computing times shrink, I expect big miners will be paid by big corporations and banks to directly deliver new blocks to them, bypassing the network and its latency. This is an example of an external incentive.