When Ethereum is secured by POS, I CAN set up another network, that WILL be just as secure as the original Ethereum network, as long as the stake holders of my Ether network are as trustworthy / decentralised as the stake holders of the original.
The ETH holders aren't decentralised at all. Especially not when the presale was heavily selfbought which nobody can prove it wasn't. So having a clone that is actually much fairer and wider distributed is really easy to achieve.