Thanks. Interesting perspective.

Miners aren't necessary in a PoS blockchain.

As for the matter of having to keep track of every single blockchain, that's not really true.
- the issue is that you'd need a way to tell that the other party has paid you, and vice versa
- since you're not on the other chain, you can't tell whether the other party has paid.
- so a decentralised exchange service could, say, set up an atomic group_sig (m-of-m) transaction between buyer and seller, and the two payments would happen as one indivisible contract.
- the chain that the exchange is built on would record this (this chain need not even be the chain of one of the coins in the tx)
- and your wallet would track this chain.

Done (hypothetically. I'm just riffing here; haven't heard from the devs yet).