Let's imagine that in your backyard you've found a well with a shitload of something which looks like gold, smells like gold, tastes like gold, and has all the chemical and physical properties of gold. With one notable exception -- that shit can be pumped out of your well in tons, at 5 cents per ton.
You pump that shit ounce by ounce and quietly sell it in small quantities to local pawnshops. But one day all your neighbors found similar wells in their backyards.
"This will be the end of the gold market as we know it!", you might say.
Nope.
To keep the market it's enough to say that your "gold" is actually faked shit, no matter how close it resembles "real" gold. Because "real gold" has "limited supply", it's hard to find and mine, it's kept at banks and bullion vaults, and simply because such dumbasses like you and your neighbors have no chance to find such a thing like gold so easily. The only question here is who will be saying this and whom people will believe at the end of the day.
With proper (not that high) IT skills it's relatively easy to clone Bitcoin's codebase and to start an own bitcoin from scratch. If no rules are changed in the codebase, the new "bitcoin" will differ from the "original" Bitcoin only in some hardcoded things like DNS pointers. In other words, it will have the same properties as "limited supply" and other shit, only run on different nodes and launched not by Mr. Nakamoto. In yet other words, it's DNS pointers and Mr.Nakamoto's name that make the "original Bitcoin" look unique. Otherwise, it's possible to have an infinite amount of "bitcoins" (with "21M limited supply" each)...
