Ideally though from a in universe reason it's existence makes sense even if there was some dubious things that went into it's creation and made complicated by the fact that a single nation basically just weaponized space without telling the rest of the world
America was in development of the atomic bomb was not announced to the world at its inception. National security is a thing for a reason.
One of the main issues though is earth will never know just how many alien threats are out their and some of them you would think would be so alien as their motivations would be beyond human understanding. I guess it does depend though on just how informed earth is when it comes to aliens. How much has Superman and Supergirl told the governments over the years I wonder?
Superman, Supergirl and J'onn are outliers in comparison to the invasion / global threats that have come to earth over the course of this series. No mature or rational mind would hesitate to seek a defense based on extremely dubious ideas of pacifism when the entire human species was/is in danger. There's no debating the scope and gravity of the alien threats seen since this series' debut--either it was a danger to humankind, or by saying it was not, then anyone taking that position just stripped those arc of any of their drama value.
It doesn't help that there seems to have been a shift in tone with aliens on the show. In season 1 aliens still felt like regular aliens but all of sudden in season 2 they are shown to be part of everyday society. It seems like sometimes they want aliens to be just aliens but then sometimes they want them to be social metaphors.
If by
"they" you mean the showrunners, then you are correct--they are generally motivated by sociopolitical concerns over
Supergirl as a fantasy adventure, so without justification, characters will make sudden turns in contract to an established characterization, all to go in the direction you identified.
It's the more alien style aliens of the shows universe that I think would be why you want a weapons like the space gun for defense. You never know if your going to be getting refuges or some "Independence Day" style attack if you don't know what is going on out in space.
Sensible. Humans cannot possibly assume the innumerable alien cultures are all some UN-esque gang of benevolent creatures, and the record of humankind-threatening attacks during the timeline of the series proves that assumption to be false and naïve in the extreme.
I don't know. We do need weapons for self defense. We have tanks and soliders and airplanes. The space gun would be just a extension of that. While I agree that before you just starting shooting at things you do try and make peaceful contact with any ships coming to earth but the fact that one might need to use it as a last resort makes sense to me. Especially in the case when dealing with a threat that can literally wipe out the entire human race.
Again--rational, but the series has drawn hard lines in the sand: the people who recognize the need for advanced security, and those who pretend there is no way to know the motives of all aliens coming to earth, all topped off by the number of threats that earth has suffered in so short a time.
There's a reason Lena, the president/Haley and Alex have all argued in favor of the human need for strength or a stronger form of defense; you could not find a collection of more disparate personalities/backgrounds and life experiences, but on this one subject, they have all made variations of the same augments, because they live in
reality--not Pollyanna-ville where no threats to the survival of humankind did not come from extraterrestrials...unless they were on some other series and not
Supergirl.
As noted days ago, their arguments are essentially the same as Fury's in the first
Avengers movie, and there's no mature counter to their position, other than some using one born of questionable sociopolitical concerns and not--inexplicably--human survival concerns in the wake of the aforementioned threats/attacks.
Maybe giving humans superpower which is what they seem to be going for but then again giving that power to the government seems to come up with some of the same issues that a space gun yet just alllowing regular people to get superpowers without regulation also feels like giving people free guns without background checks and also more dangerous because a person with superpowers depending on that power is even more dangerous than someone with a pistol or machine gun.
To be sure, the average person does not need to receive the enhanced human treatment for a number of reasons, starting with security, as a weaponized John/Jane Q. Public cannot be assumed to possess Steve Rogers-level of morality & character. The opposite should be expected--self interest, lust for power and misusing it for selfish interests (be it "absolute authority," political interests, petty resentment, revenge motives in society, etc.), so it would be applied to a select few who are--by their nature--stripped of the worst of human instincts as we know it to be in the present day with its tendency to be a knives-at-the-ready society.
Plus you still run into the problem in that the government doesn't need to own the tech. They can just recruit people with powers through the promise of money or patriotism
Again, you cannot expect Steve Rogers level of morality and character from the average person. There was a reason Erskine (
Captain America: The First Avenger) repeated the well considered ideas of who should receive his super soldier treatment and who should not--and the right kind of person is rare, no matter what walk of life they come from. Erskine did not have full trust in government (for good reason), but had he survived, his program was to be an arm of the government. His greatest stipulation was all about the character of those who would be selected..and that's just anyone on the street.