Except that everything you thought was anonymous in Monero up until that point all of a sudden is not anymore.
And if you anonymized your coins with Darkcoin before "all the masternodes are accessed", you're still safe transacting anonymously. And all your previous transactions are still private as well.
Sure, but let's be honest, secp256k1 (which is what Darkcoin uses) is
considered unsafe by Bernstein & Lange, whereas 25519 is not. In other words, if we're talking about compromising the cryptography on that level Darkcoin is at a greater risk than Monero.
I don't think it's possible to compare the two attacks (compromised masternodes vs. compromised 25519 cryptography), they're too dissimilar. Although one thing to bear in mind: if a TLA had to compromise either of those fully we wouldn't know about it until it's too late. Chances are we'd find out years later.