I say failure.
I'm sure Durocher and company will think the French results are some sort of triumph for "mainstreaming." What I see is the bulk of the French political spectrum so hysterical about the FN that they're willing to joining forces in some sort of Submission-like scenario to keep the FN out of power. And "informed" opinion in Europe thinks that such manipulation is a good thing, and "healthy" for "democracy."
In other words, the Establishment treats the FN as if it were Golden Dawn. All the mainstreaming and backpedaling and centrist nonsense has meant nothing. "Oh but it got them votes," the mainstreamers will say. Did it? Or are the votes coming from White desperation and the reaction to terrorism? How do you know a more radical political party and leader wouldn't have gotten the same, or more, votes? And if the Establishment is successful in blocking the FN from power, then what good is mainstreaming when the whole point of mainstreaming is "it'll help us achieve power and then...."
I'm not sure the mainstreaming strategy is working. And what happens if the FN does come to power and then governs in a centrist "safe" manner in order to retain that power? The point will be what then?