I think Necrons are fine without flyers. I actually have a lot of Necron models, which I just ran out of enthusiasm for buying more models for before the flyer models were released, so I've been having to learn how to survive without flyers.
Really the worry about flyers is not "do I have any?", it's "can I kill them?" Having your own flyers is the easiest solution, but if you can't or won't have your own flyers, Necrons still have a few ways to kill the other guys'.
- Fortification Quad gun
- Annihilation barges, even when snap shotting, will hit around 4 times on average with their main gun (because each 6 rolled resolves to 3 hits, and with the twin-linked rerolls you have 8 chances to roll 6's)
- Here's something I just tried out in a small tourney yesterday. A unit of 5 Harbingers of the Storm. This is 20 shots with Haywire. So it will, on average, kill even a Stormraven in one turn (unless they evade).
- Zandrekh can give an Annihilation barge or the squad manning a Quad gun Tank Hunters, which drastically increases the damage output as they try to shoot down a flyer
In game 3 of the tourney my Harbinger squad shot down 2 vendettas in 2 turns. The downside to the gun is its 12" range, but Necrons do not lack for creative ways to move squads around
As it turn out I won $40 store credit and put the winnings into... a Night Scythe kit. So I guess that starts my descent into the dark side... but anyway... I think there are tons of good Necron armies even without flyers.