The coin is secured by the blockchain.
The blockchain is secured by the miner.
The miner is secured by the block reward.
So why reduce the block reward when it's unnecessary and detrimental to the security of the system? The problem is that the coinbase has a hard supply limit and no mechanism for deflation. The solution is to keep the block reward constant until the very last coin is mined from the coinbase, and give all transactions expiration dates, a year for example, so that lost and abandoned coins can be returned to the coinbase. Expired transactions, and all previous transactions with no other dependencies, can then be pruned from the blockchain.
I think you're talking about demurrage. Well, this concept is already used in cryptocurrencies:
http://freico.in/how/But Bitcoin is not a demurrage currency.
I think the reasoning of decreasing block reward is strictly economical and has nothing to do with Development & Technical Discussion
By design, after the initial inflationary period (during which the coins are being distributed into the circulation), Bitcoin is supposed to enter a deflationary state - that's why the reward decreases in time to eventually get down to zero at some point.
To be a useful medium of exchange a currency cannot be too scarce or too plentiful. It must maintain a level of supply that maximizes it's value and velocity. That's why banks employ mechanisms to regulate the money supply according to economic activity. To simply inflate and then deflate is not useful.
Bitcoin's block reward is only a mechanism for inflation. Reducing it doesn't deflate the coin, it merely delays reaching the maximum supply. Bitcoin only deflates as people lose coins but it doesn't track lost coins in the blockchain so there's no means of returning them to the coinbase account which is what's needed to preserve the block reward.
The block reward doesn't need to be "preserved", even if 13,000,000 of bitcoins are suddenly lost tomorrow, it will continue to reward with the same 25 bitcoins, because these 25 bitcoins are issued as new coins, out of "thin air", like a fiat currency. One thing is the block reward, another thing is the transaction fees paid to the miners.
BTW, the regulation of money supply by banks doesn't work very well.