If there are no miners, then no transactions will take place within the network.
If there are no more coins to mine and only live from fees then:
1. What will keep miners from imposing high fees.
2. How many people will stay with bitcoins if miners impose high fees
This is far in the future, so there is no need to plan for this eventuality. The developers will think of something...
If you didn't buy that, how about this...
Limit the network to a small number of transactions so that fees will always be high.
Without this artificial scarcity, there is no guarantee that high fees for miners can be extracted.
http://www.reddit.com/r/Bitcoin/comments/13jj0d/in_favor_of_not_increasing_the_block_size/
The developers are all behind this idea. BitPal, here we come!
http://bitcoinstats.com/irc/bitcoin-dev/logs/2012/11/21
If you didn't buy that, they suggest charitable donations to miners, assurance contracts, and also claim to have some other vague ideas that will help.
https://bitcointalksearch.org/user/mike-hearn-2700 (see discussion from here onwards in this thread)
If you absolutely refuse to swallow the blue pill, here is the red one:
https://en.bitcoin.it/wiki/Proof_of_Stake