Does a low profit margin on mining reduce the integrity of the blocks stored in the transaction history?
All that is needed is a number of miners sufficient so that a consensus of bad actors gains control to disrupt the blockchain doesn't happen.

Transaction fees are a tiny fraction of the level of the block reward subsidy ( still not exceeding 1% ) but as bitcoin is more widely used, the fees will eventually make up for the lowered subsidy.  That's what is expected, and so far that appears to be what is actually happening:


http://blockchain.info/charts/transaction-fees?timespan=180days&amp;daysAverageString=7