However, it's very hard to "lose" a Necron. You can destroy his biometallic shell, but his "real" conciousness just downloads to another one, and returns. They are neither dying nor growing. All that they have is it. Other than the AI that's replicating into Biometallic shells. He can grow forever.
It is actually quite easy to "lose" a necron. If enough damage occurs to the Tomb World's backup systems, or something disrupts the phase out protocols, that necron is gone. And this can happen to the best of necrons, be they a simple immortal or a phaeron.
This is before you consider the somewhat feudal nature of necron politics, where many a lord will happily engineer the permanent death of a rival in order to gain a stronger power hold for himself. Even crypteks, normally responsible for maintenance of such systems are not above making accidents happen should they feel slighted by their clients.
2) We don't know what the Inertial Drives did, or how much power they take/took. It's very possible the designs are still known to the Necrons, but they need something like a captive star to power them, and the 'crons don't generate power like that. The C'tan certainly could.
The necrons are still quite capable of draining and weaponizing or otherwise utilizing the energy from stars. Even their resurrection orb technology is basically a miniaturized star in a bottle.
3) Domen Gates are easy and efficient to use. And, bear in mind, some Necrontyr are starting to deal with the younger races, and build hybrid empires. They MAY have warp travel through them. And, maybe they can discover a way to do warp travel without a Psyker, especially if they capture Tau Semi-Warp drives.
Tau warp travel is unsuitable for large scale galactic travel. Nor does it account for what the necrons used before the dolmen gates.
The Necrons have a pretty good idea what's out there for enemies to face. Perhaps their ships still have the FTL drive's but they choose not to use them because the longer it takes to reach a planet the more the competition fights itself.
But the codex states specifically that they rely on the Dolmen Gates, and would be forced to use slow moving stasis ships without them.
Something that has just occurred to me is gravity. The Imperium and eldar both use gravity tech, which makes me inclined to think the necrons could use it as well. And we know that the tyranids are also able to manipulate gravity as well by creating steep gravity wells around a planet's gravity field in order to travel FTL. This strikes me as something the necrons should easily be capable of as well.