Erchius Ghosts should become killable after The Ruin has been defeated.

In the post-game, gathering fuel to really start traveling the stars is more of a nuisance than anything else (at least until a crew of Engineers have been around for a few RL hours), and with how easy it is to make money, it seems that it's more time-efficient to simply buy all the fuel you need instead of grabbing small amounts of fuel at a time before warping away from the Erchius Ghost.

This, unfortunately, leaves moons relatively meaningless and uninteresting beyond the first bits of fuel that you need.

I'd like to suggest that Erchius Ghosts become killable once The Ruin has been defeated, with whatever necessary lore explanations being that either Nox or The Ruin itself had devised some way of making them invulnerable as a way to hamper anyone who might try to interfere with their plans.

This would make gathering fuel to be a bit less tedious in the post-game without affecting the storyline.

 

Y'know, you can just terraform the moon, then the ghosts will stop coming.