The problem with this is that it suggests that the intention, regarding the Enchantress and her brother, was, well, nothing.
So the plan was to get Waller out, then . . . let witch destroy the world?
I'm not saying that it should have been part of the Skwad's mission, I'm saying it makes the entire villain an after thought to the story.
If there was, at the very least, some other plan in action to deal with the Enchantress, while the Squad rescued Waller, and said plan ultimately fails, THEN it would have gelled better.
However, either that was never done, or if it was, it was edited out of the final film, or I COMPLETELY missed it, which is VERY probable.
For all I know, there was a throw away line in Flag's "confession," right along side the other post hock exposition that was thrown in there explaining the original plan to deal with the world ending threat.
Also, as others have mentioned, regardless of whether Waller was exaggerating about the Skwad taking on the next Superman, she still sold them on the matter of dealing with the threats other meta-humans present.
The film's ENTIRE premise is sold on the fact that they will be necessary to deal with meta-human threats.
Then, the inciting incident of the film IS a meta-human threat, but the Skwad is ONLY being used to rescue a human, and just IGNORE the world ending threat. So, the Skwad has NOTHING to do with the VERY kind of meta-human threat they were created to deal with.
The ONLY reason they, ultimately, end up dealing with it is because they choose to. Not because there was a plan that failed, and they were our LAST hope; but because there was NEVER a plan, and they just HAPPEN to still be there after getting Waller, and they are our ONLY hope, because there never was anyone else, nor were they intended to do this themselves either.
It's like everyone just wants to do their stuff AROUND the Enchantress, but don't bother her, don't get in her way, she's got her thing, we've got ours, and never the twain shall meet. It's fine, don't worry, her plan will never work, and she'll eventually calm down and go back to her cave.
I'm FINE with the Skwad being, ultimately, used for a smaller, more covert, "rescue" mission, and then finding themselves the only ones who can save the world. That's a FANTASTIC concept for this kind of film. But you can NOT just have had the world ending threat just THERE. A nuisance that everyone in the film otherwise ignores, or just try to avoid. You also can not pitch the team, sell it, SOLELY on the notion that they are necessary to fight the precise kind of threat you then proceed to completely ignore.
It's one thing for Waller to lie to the government about her intentions for the Skwad, as she always has her own agenda, and motivations, etc. But you then need to address that when the lie she sold them on shows its ugly head.
Her higher ups allowed her to proceed with Task Force X on the basis that they would be needed for meta-human threats. Then, when a meta-human threat presents itself, they all agree to activate the Task Force, but NOT to deal with the aforementioned threat they were created to deal with in the first place!
They're just going on a little tangential side mission, while everyone just waits for the swirling apocalypse in the sky to sort itself out!
I had guessed that Aquaman joining would be a major part of the film itself. Recruiting within the film itself would be a way to do things differently to Avengers. 
