You think when it all comes down to it and the full Tyranid fleet is in full gobbling force, will Necrons be the last line of defense?

Their completely inorganic state makes them ideal for combatting Tyranids along with their highly evolved weapons that seem to be..customized to fleshy things.

I don't really see any of the other races, save maaaaaybe Orks for sheer Horde vs Horde action, putting up a real fight for long.

Honestly I think it depends on what direction they decide to go with the Octarius ork nid war. They may write Ghazy as a great big badass who suddenly decides the best fight is vs nids. I could actually see them ending his story arc with him building up a super badass ork force and begin pushing back in the direction nids came from.

Nids would still slip past and therefore remain relevant in the 40k universe but their overwhelming force could be stymied to the point of not being the end all doomsday they currently kind of are.

I would say it is certainly plausible that the Necrons could hold the Nids at bay considering not all of the tomb worlds have awoken yet and it is believed that there are billions of necrons still slumbering even after the great awakening.... So there is still plenty of room to add and modify necron tech to make them more resilient to the tyranids in the lore.

There is nothing in the lore that I have seen that said the Tyranids can't consume the living metal that makes up Necrons. A Tyranid's Bio-Acid can break down many inorganic materials including minerals, and Necrons are ultimately made up of minerals, now we may not know what kind of alloy or minerals the necrons are made out of but, if there is any Carbon, Iron or Calcium (to name a few) in their bodies, these minerals can be harvested and converted into biomass as these minerals can be found in humans and we all know what happens to humans. When Tyranid Hive Fleets consumes a planet, they remove EVERYTHING down to the molecular level. So they may not take all of the Necron but they certainly take some of them. Furthermore, there is nothing saying the Tyranids can't evolve to create a bio-acid specifically designed to breakdown the Necron living metal, all the Tyranids need to do is breach and take over a few Tomb Worlds (no easy task but is doable) to amass enough of that metal to learn how to consume and overcome the regenerative abilities of the living metal.

The biggest challenge the Tyranids face is catching the Necrons. The Phase Out Protocol is what makes the Necron so formidable as it helps maintain their immortality and denies the enemy their ability to completely destroy the Necron. If a Necron can be caught, and cannot phase out, it will either be destroyed through consumption or through the Self Destruct Protocol which permanently destroys the Necron.