Why not decrease gradually but suddenly cut by half?
It's simpler this way, and will be less disruptive than might first appear.
Is there anyone can change this? As I understand, this is not part of the hash calculation and block generation but rather a system design
Only if he can convince all Bitcoin users to switch to a software following his new protocol. And it is generally agreed that the coin generation schedule is one thing that must never be changed, so no.
Actually I still have no idea how those 50 BTCs are generated and send to the one who found the block, is it generated out of thin air when the block is found?
Yes. Remember that coins aren't physical objects that are created, strictly speaking they aren't even digital objects that are created - they're just an agreement between all nodes on the network that address X is entitled to number of coins Y. This agreement is written in the block chain, which can be viewed in http://blockexplorer.com
. There are rules that specify how bitcoins are moved and generated, and the rules say that anyone who finds a block can specify a coinbase transaction which adds 50 new bitcoins out of thin air to an address of their choice. If they try creating more than 50 the block will be invalid and everyone will ignore it, and it will not become a part of the block chain.
Who generated it, what if the finder does not even have a bitcoin address? Must any bitcoin server have a bitcoin address?
That's more or less like asking what happens if an employee doesn't have a bank account, working at a place that only pays salaries to bank accounts. If he wants to work for free great (might be illegal in the real world), but it makes more sense to have an account and receive payment. Mining is serious business and no one will want to do it for free.
If mining for a pool, the pool basically tells you where to send the payments, and you tell the pool where to send your part of the loot.