It wouldn't necessarily need a complete redesign to shut that attack vector. All that's needed is to prevent the mixing masternode from knowing which outputs (of the mixing transaction) belong together with which inputs. The mixing peers could do a key exchange and encrypt their own outputs with random mixing peer's public key, and decrypt the others' outputs they can using their private key, and then pass clear text outputs to the mixing masternode.
Or they could do all this without masternodes at all, as is being done with Bitcoin-based mixing systems, so again, Dash and masternodes are pointless.
Difference is, that the mixing in DASH is automatic, trustless, and decentralized. And, mixing is not all that masternodes can do. Instant transaction confirmations come to mind. Further, I wouldn't say having a healthy number of full nodes is pointless.