Can a system like Ripple exist without a currency like ripples? As they are not used to pay the peers in the system, and their supply / fee ratio is so large they do not appear to be stopping the system from being spammed, could the system work without them? What is stopping the Ripple system from abandoning ripples, asides them being mandated by the protocol?
Not with its consensus mechanism: you need the scarce ripples to prevent abuses to the network precisely from spam and DoS.
But there is the older two phase commit design to implement the ripple concept as a decentralized protocol (that wasn't implemented) and didn't need ripples or any kind of "HostCoin":
It has some advantages and disadvantages over the current implementation:
+Even faster transactions??
-All intermediaries in a Ripple transaction must be online
-Lack of thin clients, people that can't be always online are expected to use a server as a service
Privacy is not a priority for the current implementation right now, but there's plans to improve it through obfuscation.
The ledger could even act as the "register" for "hidden transactions" more similar in properties to the ones of the old 2pc protocol. So it may be more convenient to extend or modify the current protocol than to implement the old one.