It's still possible Ash loses to Olivia, and has to take on alternate trials to re-challenge her.I hate how this is happening, but I have to disagree, now that I really think about it.
IF Ash was NOT going to get the water type Z-crystal, but indeed JUST Lana then I'm perfectly fine with what happened Ash not battling Wishiwashi (especially if translations of the episode are accurate when it was Lana who earned the right to battle Wishiwashi). What I am not fine with, is that it seems unlikely Ash is going to get an alternative trial (AKA a different Totem Pokemon from the games or at least a different Z-crystal from the games), it's not absolutely confirmed Ash won't get an alternative trial I just have SERIOUS doubts, and it'll be too little too late if it does end up popping up.
Furthermore, USUM is highly likely to feature new trials, so what better to include alternate trials in the anime?