And why would any native clubs and their fans would accept it? That's against fair play to the very core of that principle... And since pro footballers equal to workers, this would like seeing you have to suddenly compete at a duel against one of the most elite in the business to stay in your job after you previously on smooth sailing at your job and have earned promotion. Would you accept it? I surely wouldn't.
Unless of course if bribery involved, simply put, there is no way that can work in any fairness principle.
And what if by the way of chance or miracle barca didn't win the play off?

throw them to 2nd division? Lower?

you have opened the pandora box with that plan.
The only way this can happen is if barca goes to MLS or a club went bankrupt or disbanded so barca can take the place, still that goes back to my question, why they instead of the more deserving native clubs? I sure wouldn't accept it if I were them.