At a point in time, A Necron army was something to be reckoned with, and indeed as time has moved on, unsympathetically to their fluff, they have not aged well. Sure in the storyline they aged remarkably well, and are perhaps the single most powerful force in the galaxy, but dammit if they aren't underpowered on the battlefield. So is a new codex coming? That's the idea. It certainly seems like the right time to update them, but then it would be very unfitting of Games Workshop to do something at a time when it would be most well received.
The truth about the Necrons is this: The reason people still play them, and the reason they are still relatively popular is that they host something very dissimilar to the average Warhammer 40k army. "We'll Be Back" grants the Necron player a specific psychological edge, despite the ability being arguably less powerful than feel no pain.
Feel no pain is great, and we'll keep more Necrons alive in the short run, but the psychological winds of change that can come about from a portion of your Necron army returning from the dead a turn after they had been blast to pieces can cause frustration, and can make even the best opponents make tactical mistakes, because there is no telling who and who will not be back at the beginning of the next turn. That psychological edge makes us potent where we would otherwise be shit.
I hope with all my heart that Games Workshop does not make the mistake of turning WBB into FNP. It would be devastating on a level great enough to ruin the army for its long-time followers and players.
Necrons deserve a legitimate update, not a throwtogether leap into 5th Edition.
If they refuse to do it properly, I'll just play the 3rd edition codex long into the end of 5th edition, and you can all suck my C'tan's giant scythe.