What really matters is whether the users want to own the new coins or the old...It's also important that the new coin is a fork of the old, because basing it on a snapshot of Bitcoin's block-chain means the value of users' holdings in Bitcoin is preserved in the new coin.
Whether you create a fork or a spin-off, the end result is the same in this respect: in both cases you have bitcoin (PoW) and bit
shares (PoS) running side by side. If you want bit
shares to supersede bitcoin, then you need to legitimize it somehow and the market needs to agree.
We should also agree on the definition of a "fork" and a "spin-off". Here are my thoughts:
FORK:
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A fork preserves the complete chain of digital signatures in the bitcoin blockchain back to the Satoshi genesis block with no missing details. Bit
shares implemented as a fork would mean that new PoS blocks are built forking out from some pre-defined block #
X and that client nodes would likely download all new bitshares blocks plus the entire bitcoin blockchain up to the forking point.
SPIN-OFF:
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A spin-off preserves the wealth distribution (provable via ECDSA private keys) as specified by the unspent outputs in block #
X but does not necessarily preserve the complete chain of digital signatures back to the Satoshi genesis block. Bit
shares implemented as a spin-off would mean that new PoS blocks are built on top of some "nucleus" that represents a snapshot of the blockchain's unspent outputs at block #
X. Client nodes would likely download only the bitshares blocks if the nucleus was hard-coded (the unspent outputs require vastly less disk space than the blockchain transaction history).
By this definition, this project is intended to be a fork, indeed what bitcoin core developers call a hard fork.