We don't know what the cost of transactions in Bitcoin will be yet. Bitcoin includes two methods to pay miners for the infrastructure costs of running the network: mining rewards and transaction fees. The idea is to bootstrap the network off of mining rewards, and then switch to relying on transaction fees in the future, as Bitcoin reaches its limit of 21 million coins. The problem is that mining rewards are currentl…
You're assuming that mining revenue must never decrease, but there's no reason for that to be true. A more likely scenario is that as block rewards go away, instead of transaction fees going up, overall mining revenue goes down. Some miners will exit, but as long as there's money to be made there will still be miners. Bitcoin users and miners will adjust fees until they reach a compromise that's mutually acceptable.
Part of the genius of the block award being so top-heavy in the beginning is that it made the network much more secure than it would have been if it had relied on transactions from the beginning which will allow it to mature and stabilize before supply & demand determines what the market is willing to bear in transaction fees.
In addition, it allowed a decentralized method of distributing the coins that isn't able to be gamed.