Biggest owner wins. That's why it is not trustless. The only long-term game-theory outcome for the system is a monopoly. Owners own the miners, and dictate the rules to the users. Otherwise known as our world financial system.

> In the POW case, what underpins the value of the coin?

The network effect of the decentralization, i.e. the nodes. It is the nodes that are peers in the peer-to-peer cash of bitcoin. It is they who hold the blockchain, validate transactions, and validate blocks. Because there are so many of them (185,000 and counting) unless you can effectively convince all of those nodes to adopt consensus changes, the existing consensus rules apply.

PoW alone doesn't make something valuable. Its successful implementation does.