No enough attackers with the know-how, not enough LOLs in it for them and so on.
But once bitcoin built its super hashpower and implemented merged mining new coins no longer need face that if they have even merely just enough of a good idea to get a merged mining pool set up ready to add them to its merge at launch.
Plus now, even better, supposedly proof of work is no longer even needed to secure a distributed p2p ledger so anyone can easily issue a new secure currency simply by cloning a secure no proof of work required system instead of the insanely expensive to secure, insanely hard to secure old proof of work concept. Maybe clone Ripple, or NXT, or whatever of those things does in fact work, if any of them do in fact work.
Either by merged mining or by some newfangled no proof of work needed system, launching a currency that is secure from the get-go is easily do-able nowadays.
-MarkM-