There aren't enough use cases for bitcoins right now which would support either a large number or a high enough transaction fee. At least nowhere near the payout of a 50 btc reward for confirming a block today. We can safely ignore the influence of fees -- several pools keep the fees and only split the reward and nobody cares very much about it.
We're a LONG way from not having bounties on Block confirmations. Next year some time, it will get cut in half (25 BTC), but assuming BTC haven't crashed back to the sub $0.05 level, that will still be enough to keep miners interested. As BTC grows, the number of transactions will grow, as people start using it, they'll want their transactions confirmed quickly, so they'll include transactions fees. As the number of transactions appearing in a single block grows (the transactions per second (tps)) the fees will increase.
If it all works out and BTC is successful, the transaction fees will FAR outweigh even the current 50 BTC bounty. However, at that stage (2030?) there will only be professional miners, because the hashrate will be so high that casuals won't be able to compete.