Depends if the galaxy/universe turns into a warp rift. If it does... even the crons would be overwhelmed, and the nids would go into Nurgle's soup. Remember it would mean unlimited demons. Only the Emp's sword can kill a demon.
Valid, but the extent of the Necrontyr pylons scattered throughout the galaxy is currently unknown, but implied that it's very expansive. The Necrons biggest weakness is that the vast majority of them are still asleep, so the likelyhood that they could activate all the pylons on their own in a short period of time is probably unrealistic. Some kind of alliance between Guilliman and Szarekh could facilitate that though. The Mechanicus already has a knack for awakening tomb worlds, so what if they did it on purpose instead of on accident? It's also canon that the Mechanicus is technologically capable of operating the pylons when instructed by an overlord.
Additionally, the Necrons control a number of superweapons that they don't even allow themselves to use because they're so destructive. If all is lost and the universe is getting consumed, what is stopping the Necrons from walking into the orirary and wiping out 90% of the universe? It would be a massive setback for reversing biotransference, but still a better option than losing the universe to Chaos. An empty universe is better than a lost universe, because Necrons are capable of waiting the billions of years needed for new planetary bodies to form and suitable lifeforms to evolve. You can't forget that Necrons are a species that write and perform plays that last for tens of thousands of years, just because pseudo-immortality can get really fucking boring. I guess it beats getting so bored that you fuck a new Chaos god into existence.
Either way, it feels like the Imperium and the Necrons are being put in a position where an alliance makes sense. Some important Necrons don't want humans eradicated, and the return of Szarekh implies that the dynasties are about to be unified under his rule. If he doesn't want humans eradicated, the random rogue dynasties will stop attacking the Imperium thanks to command protocols. The imperium is getting bent over backwards by Tyranids, and Necrons are the ultimate counter play. The secondary threat is the impending death of the emperor, and if he dies, Chaos takes over. Chaos isn't afraid of Necrons in the same way Tyranids are, but they are immune to influences of the warp, and they have technology capable of manipulating or outright controlling the warp from the material plane.
Other possibilities in favor of the Necrons is that Orikan could master transference and become an unshattered C'Tan. That would potentially put his power level on par with the emperor and Chaos gods. They also have access to the webway, so if shit goes South so fast that they can't react to it, they always have the option to run. They don't need to kill anything when they have the ability to prevent anything from leaving the warp. If they were successful in doing so, it seems like the powers of the Chaos gods would be massively diminished due to their inability to corrupt.
I'm hardly a lore expert, but I have read most of the Necron books and they're probably closer to the role of anti-hero than anyone else in the galaxy, and they have shown a willingness to help the emperor when their goals overlap. The only thing really stopping them from working together is the Imperium's zealotry. Higher ups like primarch and inquisitors have the information and the guts to interact with them on an individual basis, but if the average human knew there was an alliance, it would cripple the ideological principles the ecclesiarchy uses to keep the imperium under control.
