Wouldn't Flextrans have the exact same problem? I haven't studied Flextrans in detail, but from what I remember it would enable a new "version" of transactions without malleability. But wouldn't legacy transactions ("v1", as they call it here) continue to be allowed in this proposal, too? In this case it could lead to the exact same situation where a malicious miner or pool could try to spam the network with legacy transactions to "take out" some competitors.