If you are a target, the attacker can just create some mixing transactions and mix them with you. The masternode will do the actual mixing, but the sybil attacker knows his own mixing transactions and thus can deanonymize you fairly easily. This is certainly true when the liquidity/activity (e.g. only a few Darksend transactions per hour) in the DASH mixing network is low. Liquidity providers don't really help, because when they are spied upon (or forced by a TLA to give them a backdoor), the spy (or TLA) can see all the mixing logs and can connect the dots trivially. I want to mention here that a lot of masternodes are hosted on cloudhosting servers. Therefore, the OPSEC is in the hands of the cloudhosting companies. In addition, as far as I know the liquidity providers are known figures from the DASH community which also makes it easier to connect thet dots.
Wildly implausible scenario. Your entire argument hinges on the fact that the attacker must know exactly when I'm about to mix (which they don't)
and have access to every hosting provider with every Masternode because they can impossibly know which Masternodes are going to be used for the mixing process. Just one node off their radars and they can kiss their attack goodbye. You expect anyone to believe that an attacker, even nation states, can get access to every hoster in the world with Masternodes on them? What about people running private Masternodes at home? As I said, your attack scenario has nothing to do with reality.
Also more important differences.
Can they de-anonymize anything after the fact in Dash? No, because it happened off-chain.
How about Monero? Yes, retroactive exposure will always remain possible.
Why is the XMR blockchain so bloated? Because due to the insanity of default anonymity the only way to prevent double-spending is to keep track of
every freaking spent key image, thus creating basically a second blockchain within the blockchain.
I said it in the past and I'll say it again:
Optional Anonymity >> Mandatory Blindness
Retroactive-proof Off-Chain mixing >> In eternity vulnerable on-chain mixing
