Let's walk through Superman needing to form the Justice League.
Under what premise would they join with this all powerful, all good Superman around? If you try and write that script it is paint by numbers - big bad comes to planet, quick let's form the JL and fight! That would be just boring as bat s*** boring. And if Superman is this all good righteous person, then every conversation he ever has is he is always right. Again, under what reason would any member of the JL be interested? The dynamic would be so stupid and childish and it'd be an even weaker movie than the Avengers.
Let's walk through the current scenario. 2 times the world has now faced extinction level events and both times saved by Superman. Superman is no longer here. There is now a genuine need for the metas to form. For the 2 years prior with Superman around the metas stayed hidden, no need for them, they go in hiding. With him dead, it's time for them to come forward especially now with Superman having pathed the way for the acceptance in the world. Superman took the whole weight of the world on his shoulders to allow the other metas to come through and be embraced, because he is the only character than can do that. And he did.
Just keep doing what you're doing Zack, you know better than the fools who vote for Trump.
Before I present a way in which Superman founding the Justice League works (as it always has), let's address your presuppositions: you understand neither what actually happened in Man of Steel nor what the Justice League is. First, Superman had a LOT of help in Man of Steel, and by sheer luck he didn't face all 20 or so Kryptonians at once. He's not all-powerful, as his death shows. He was nearly killed by a mortal man, after all. Second, the Justice League is not merely a defense force, and Superman is not merely Earth's first responder. This is what Superman is supposed to be about, according to Man of Steel:
Jor-El: The people of earth are different than us, it’s true. But ultimately I believe that’s a good thing. They won’t necessarily make the same mistakes we did. Not if you guide them, Kal. Not if you give them hope. That’s what this symbol means. The symbol of the house of El means hope. Embodied within that hope is the fundamental belief in the potential of every person to be a force for good. That’s what you can bring them... You will give the people of earth an ideal to strive towards. They will race behind you, they will stumble, they will fall. But in time, they will join you in the sun, Kal. In time, you will help them accomplish wonders.
I think that speaks for itself. Consider that Wonder Woman isn't just an anti-Nazi feminist warrior but an ambassador on a mission to bring peace to the world. Or consider that we're talking about a group called the
Justice League. They aren't the
Metahuman Defense Force or the
Superhero Squad. These characters and this group are bigger than that.
Superman's legitimacy and the League's significance don't come from how powerful they are, but from their mission. And their missions are hand in glove. Superman wants to make a better world. The Justice League fights for Justice in the world. Now anyone can come up with a context in which an overwhelming threat forces these heroes to band together. Any team can be vulnerable and outmatched against a more powerful foe. But if you want to depict the Justice League, you've got to get to the heart of why this is a justice league. And the best way to do that is to take Superman and Batman and Wonder Woman and have them realize that the best way that each accomplishes their own missions is to form this group. Again, how you fit that into the context of a threat-based story isn't really that hard, once you have a firm grasp on the Justice League itself.
So this has absolutely nothing to do with Superman's power level. Because you do realize that your issue with Superman's invulnerability doesn't go away after the League is formed, right? The only way to resolve that issue would be to kill him off permanently. But then you really don't have the Justice League, you just have Batman's metahuman Avengers team. And all you are saying is that you actually don't really like Superman at all. But plenty of comics have had Superman as a member of the Justice League and yet facing difficult challenges. It is possible. At least 50 years of comics prove that to be the case.
On a final note, I'm not sure how you've come to equate critics of Snyder's approach to Trump supporters. This is just an Internet forum, so you won't get any points for baseless comparisons.