We've already discussed incubator tech in enough detail that I'm sure firns made them about as dangerous as they're liable to get.
I'm also pretty sure that power level is between "they own us" and "1 Neanderthal vs. Entire U.S. military arsenal + 2 Neanderthals"
Maybe, maybe not. Just because the Incubators have advanced technology doesn't mean that they have assets in place to project force here on Earth. We haven't seen any indication that they have a ship in orbit or a platoon of killbots waiting in a warehouse downtown. To me, the fact that Kyubey never takes any direct action himself would indicate that these furry telepresence units are all the assets they have on the planet, and that sending combat units to Earth would be a non-trivial endeavor.
The Incubators reached the absolute end of the technological curve, then invented fucking magic to build more tech curve to follow.
Their civilizations most pressing issue is that entropy exists - they have no concerns other than the heat death of the universe itself. They have, for all intents and purposes, solved science.
Again, I have to note that we only have Kyubey's word for this, with no independent corroboration whatsoever.
Also, every indication is that magic is a bit of a "black box" for the Incubators that they can't really understand or use themselves. Which suggests that magic was probably something they discovered by accident.
If they really wanted to, Kyubey (assuming he really is a Time Lord-esque entity) could've Killsat Madoka before she finished her wish.
That assumes that he had a Killsat ready and waiting at that exact moment. As I've said, we've yet to see any evidence that the Incubators keep space-based assets in orbit over Earth.
The fact that, when Kyubey wanted Kyouko dead, he had to mislead her into killing herself rather than just taking her out himself, indicates to me that he does not have the capabilites in place to kill any girl he feels like.
...because Kyubey doesn't cancel Madoka's wish nor does he try to impede her or talk her out of it...
He
does try to change her mind. "Are you
trying to become a God?" definitely strikes me as an attempt to dissuade her. She stands firm and refuses to change her wish.
As for him not aborting her wish, the fact that he doesn't when he had ever reason to indicates that he can't. Whether this is because of some sort of programming limitation or whether he literally can't because the wish-granting process is a black box that he can't truly control is unknown at this time.
If that's not time for him to snipe her with his Omega Civilization Supertech, then how the fuck is he concerned with Heat Death?
Again, having the theoretical technological capability to do a thing does not automatically imply that the assets to do a thing are in place at this particular moment. We don't even know if the Incubators have a standing military, much less how many millions of lightyears from Earth their nearest fleet elements might be. And their "Kyubey" telepresence units appear to be entirely unarmed.
If we're looking for reasons why Kyubey does not do something, I find it easier to believe that it's due to a lack of capability rather than due to ethics. The simple fact is that Kyubey has not
demonstrated the existence of any ethical limitations. He has
claimed certain ethical limitations on occasion, but his actual actions prove those claims to be false.
We won't find the holy grail by science farming.
There's more than one holy grail here. We need to find a long-term solution to grief build-up, because the supply of Grief Seeds is going to dry up once we stop girls from witching out. Experimenting with our powers might hold the answer to that.
It also might be possible to set up an Asanaro-esque anti-Incubator field around the whole planet. Or a containment field around their homeworld. Magic opens up possibilities that they may not be able to counter no matter how good their technology is.
And I'm going to continue to keep Grief-formed FTL drive and Grief-saboted relativistic projectiles in mind as a weapon of last resort. Our powers don't seem to be subject to the physics problems that such a thing would usually entail.