Or what if once he's big enough to be a landmass, people start living on him? He would always have company then.

And yes, he'll outlive Luz and Eda, but not the collector

Finally someone else mentions this here besides me.

King will most likely be so huge that he will become a continent himself - it will probably take years/decades/millennia (?) but in my opinion it will be like that.

And we also learned in canon that King was still in the egg when he heard his father's scream - centuries/millennia (?) later Eda found him when the Boiling Isles were already living on his father's body.

So this could be interpreted as meaning that Titans also have a very long lifespan because hatching seems to take a very long time, so it could also be that King will outlive everyone except for The Collector.