I'll believe it when I see it, as there are still countless barriers to overcome, the most prominent of which are:
1) Convincing the carriers who have just been moved to T5 to move again. It'd be possible with LAWA muscle if AA held the lease for T5 as Delta achieved with T2/T3, but to my knowledge this isn't the case with JetBlue, Frontier, or others in T5 who directly lease with the airport as opposed to sublease with American, so they thus have no obligation to move unless AA can provide them like for like facilities at midfield
2) Getting the entire plan through the EIR process with both El Segundo and Westchester. For AA to capture the rest of the gates at T5 would mark a significant expansion effort - one that the surrounding communities would be keen to shut down
3) The standard CBP and TSA woes that are already going to bring the airport to its knees this Summer
(1) JetBlue, Frontier, and the other airlines have no leverage. None of them has an existing need to lease the majority of gates in T5. None of them could finance the further improvements that T5 and the area between T4 and T5 needs.
(2) The surrounding communities have already given LAWA permission to build the MSC, T0, and T9. As long as LAWA puts the other airlines in one of those terminals, the rest of the EIR is pretty much perfunctory.
(3) What LAWA and AA will do with the T4.5 project is consolidate and expand the T5 and T4 TSA checkpoint. It's a solution, not a problem.
There is nothing to believe. The two parties have signed a letter of intent. They have a plan to overcome all the barriers you think exist.