She's never been DC's first superhero though, and giving her that status has never been beneficial to her or to Superman. It will undermine the impact of his arrival because she gets all the fanfare, or more likely them come up with some unconvincing reason why she DIDN'T have much impact and acted low-key so as to give Superman his big entrance.
They need to be peers and debut around the same time if we are still doing the shared universe route. It's the only way to pay proper respect to both of them at the same time. If they go with the unnecessary model of having her with the JSA, someone between her or Clark is going to get screwed over. It will most likely be her, but ideally it shouldn't be either.