Unless 'it' left a long time ago and is already here.
I always feel the 'space is big haha no one can cross it' argument is weird due to always framing it in our own frame of reference. What if the hypothetical 'thing' is like a jellyfish and time isn't as big an issue, or an AI that definitely doesn't care about time. Or a bundle of fungus that doesn't necessarily care one way or another.
Now we've been sending radio signals for several decades you have to begin to question if there's a civilisation out there maybe 10-100 light years away that may now be aware of us and have sent something our way which may be about to arrive.
There is an explanation for Fermi paradox that suggests maybe alien civilisations choose to be quiet or immediately wipe out other transmitting civilisations with speed of light weaponry.
Why? Well, if there's no way to have prior warning of an attack traveling at the speed of light your only defence is some combination of remaining hidden and extreme hostility towards other civilisations.
It’s just that when you return, at least 2000 years will have passed on earth.
This site also has a "Newton's universe" mode, which pretends that relativity doesn't exist. Interestingly, in that case you would take over 44 years!
[0]: https://www.omnicalculator.com/physics/space-travel?c=EUR&v=...
edit: souce as in something a regular average joe can make sense of haha
More broadly, the series Cosmos with Carl Sagan was excellent, and is worth a view. The bit about time dilation is at https://www.youtube.com/watch?v=kWgEzxi_A0w . The more recent one with Neil Degrasse Tyson is also good (but while I enjoy both, I think I prefer the older one)
Might be posting on HN right now. Never know.
All your hypotheticals are not entirely unreasonable, but they are also so far beyond our current ability to test, that it becomes pretty futile to speculate, except as a source of entertainment.
Space is reaaaaaaaaaaaaaally big, much bigger than people really have a conception of. Bigger than I have a conception of, and I try really hard to understand how bad my conception of it is.
A space fungus from 120 light years away would presumably only reach us by almost 100% chance, which is something I am comfortable saying is almost 100% unlikely. This is true of most hypothetical threats.
Fantasies about aliens or AI coming here (or sending big rocks) to exterminate are as weird to me, as I image "space is big haha" is to you.
There is no reason for anyone even to try to cross interstellar distances, especially not for war. The expected gain will never be worth the cost, outside of some fantasy FTL-for-free engine, which is just that: fantasy.
But, it is fun to think about it all.
Takes a few million years to set up... but that's just a blink of an eye on cosmic time scales; a brief era in the galactic-scale lifespan of an advanced civilization. (The civilizations that don't do something like this don't live that long).
Annihilating planets takes a lot of energy. For example if our Sun goes supernova it will maybe kill all life on Earth, but the planet will still exist.
If you gather enough energy to potentially annihilate a planet in each star system in a galaxy - you won't need to worry about any civillization becoming a threat in the first place, because they won't have any energy left to become that threat :)
Then there're the problems with relying on autonomus system with such energy at its disposal, and the problems with dealing with genetic/cultural drift. What if your species diverges into several ones that start fighting each other - which one is the one that's supposed to be protected from the other? What if different parts of this system disagree?
Who gets to push updates to the code of this system? How do you supervise this? Imagine a military coup or constutitional crisis and you're the guy that keeps the ssh key :)
Are distant colonies that get independence and develop on their own a threat? Would the regions that want autonomy agree to this? Or if this technology exist - wouldn't it be used for wars inside that species long before they can start to worry about the so-far-uninhabited rest of the Galaxy?
Would they even survive long enough to create this?
Imagine Sumerians created such system on Earth and it wakes up after 5000 years and asks us for password in some dead language or else :) Now change that to 1 000 000 years.
In my opinion it's very unlikely something like this ever gets created simply because of all the coordination and trust issues, the energy and technology dependencies, the probability and severity of problems it can cause vs the problems it can solve.
Wasting significant percentage of energy of the galaxy and risking destroying it accidentally just to save yourself some diplomacy sounds like the ultimate tech solution to people problem :) And you're not really avoiding the diplomacy, you're just doing it upfront - internally. I'd be very surprised if there were no wars caused by someone trying to implement such idea.
Imagine writing a proposal to Congress or EU to start doing this :) And how Chinese and Russian would react.
It's taking a few million years to get from violent confused simians to violent confused simians with autonomous AI. But we're getting there.