It has more features to offer?

Not an exact clone but something that serves the same purpose - a digital currency. If an alternative became popular enough (for example, a special version for the chinese market), wouldn't Bitcoin have to give a market share to that alternative, and wouldn't sellers want to accept payments both in BTC and in the chinese alt currency? This way, there would be more digital money in the market and their value would have to drop.

I think the problem with your comparison is that Bitcoin is not like gold - you can't create gold but you can create any number of Bitcoin clones and they are just as good as original Bitcoin.

Maybe creating Bitcoins is like mining. But not creating Bitcoin clones - this is rather like creating a truck of gold out of thin air.

You are more than welcome to take the open source software and create your own network.  With a bit of marketing you can probably make it worth something.  BTW, there is another bitcoin network, the test network, and people (developers) do trade bitcoins but they are worth a lot less.

Yes, but any of the clones will not have the same trust and good publicity as the original one.
Less trust = less value.

What your saying is very similiar to "What would happen if i mixed some chemicals (or manipulated matter at subatomic level) and produced something that looks, smells like gold and has every other property of gold".

Nothing special would happen. Depending of how scarce is the thing you are producing, and how easy is it to produce more, it would have bigger or smaller value. The harder is to produce a thing = the more scarce that thing is == the more value the thing has. Also if it is easy to divise, shape and carry, then it can be used as a currency. This rule works with everything.

It's only a matter of time, I think, that a corporation would create an initiative to copy Bitcoin.  But, I think Bitcoin would remain competitive since it was the first.

Here is a question for you.  Has any corporation copied bittorrent?

Bitcoin is a P2P technology like Bittorrent.  If companies want to control the currency then Bitcoin is not for them. 

Yes, I agree, but we are talking about a currency here. The currency's value is not that it's decentralized but that it can buy you something.
Some time ago all money was decentralized as well - if you had gold and silver coins you didn't need any central authority to tell people what is the value of your money. The gold coin was worth as much as the gold it was made of, the same with silver. Then money was centralized -you have a piece of paper with a number on it and the government's job is to convince everyone that the piece of paper has a certain value, much greater than the paper itself. So, this is still money, even if the 'underlying architecture' has changed, and still people want to (or have to) use it.

But this diverts quite a lot from my original question which was: how new digital currencies will reduce the purchasing power of existing ones?
maybe if the cryptocurrency's purchasing power will equal the real cost of creating it it won't matter how many new currencies are created, their value will not drop below the cost of creating a new currency unit? Just like the real metal coin - it will never cost less than the metal its made of. (yeah, I think I got what moa was talking about)