And how do you propose that they create more BC than 21 million? I'm not sure you entirely understand the method which BC utilizes to prevent itself from being copied.
Even if they could copy the BC code, it wouldn't work for the reason that BC is not just a normal program. It's a PROTOCOL and that makes all the difference. A protocol is like a language. If you speak English and modify a large number of words, it won't be English anymore. That's why you can't add more BC to the network if you have a similar digital currency, but with different code.
And what if you copy the BC protocol in EXACT DETAIL? Then the protocol is programmed to automatically revert to the largest existing version of the blockchain. The blockchain is hardwired in BC's DNA. The BC protocol (assuming same version number) only accepts ONE version of the blockchain, it cannot accept any other. It would simply revert to the existing blockchain and would be forced to adhere to the 21 million limit.
The only way people could add more BC to the supply is if the MARKET accepted a close BC copy and pegged this new currency to have the same worth as BC. But how likely do you think the market would accept such a proposition?
I won't say the above is impossible, but I'm pretty skeptical of a free market accepting such a notion without a fight.