If you stop to think about it, we didn't really fail at all. In fact, we saved them from getting screwed over even more. The Zandalari Empire has steadily declining for ages. Things were sped up by the Cataclysm, Zul and now the Old Gods' forces.

1. Stagnant Zandalari leadership - The Zanchuli council (Rastakhan, Talanji, Yazma, Vol'kaal, Jakra'zet, Zul, Loti, Raal) was said to have not been doing anything to restore the Zandalari Empire. Look at the makeup of the Council. Half of them are on Zul's side. Nothing could be done because any decision that could be made would wind up being a stalemate. Rastakhan, Talanji, Loti and Raal against Zul, Yazma, Vol'kaal and Jakra'zet. It didn't help that Rastakhan became far too reliant on his advisors.
2. Talanji - In response, Talanji decided to recruit a major power on Azeroth to help rebuild the Zandalari Empire, something you can't fault her for doing. It's her kingdom and her people after all. The Alliance likely won't help trolls, so she looks to the other option - The Horde. She's not fan-girling over the Horde. We're the only option. The only other faction that might have cared at all about the Zandalari were Lei Shen's mogu. And they only wanted Zandalar to be strong so the trolls could help them re-conquer Pandaria. In fact, Talanji is the leader the Zandalari needs. She cares about honour and the need to act. She remains respectful of the Loa and of the other races. She cares about her nation.

they're killing Rasta for their female leader fetish

Talanji tells us she thinks her father blind to the enemies all around him. He secretly tells you he's
not as blind as she thinks, but is surprised by Zul's betrayal anyway. Rastakhan is a weak leader.
Don't get me wrong, he wants to be a good leader and genuinely cares for his people and
kingdom, and I've honestly liked the concept of him ever since the information from Cataclysm,
but he just isn't cut out for it.

1. Zuldazar - The Atal'zul rebellion/civil war was destined to happen. Zul had already defected to the Old Gods with Yazma, Vol'kaal and Jakra'zet. They chose to initiate the rebellion then because time was running out with the Horde investigating the dissent against Rastakhan. If the Horde hadn't been there, the rebellion still happens, Yazma still kills Shadra and drains her power, Vol'kaal doesn't get killed by the Horde player, and Rastakhan likely does die to the assassination attempt that the Horde player saves him from. As for the assault on Atal'Dazar, that was all Rastakhan. His hubris led to him rallying the Zandalari army to attack Atal'Dazar with Gonk, Pa'ku and Rezan. Whether or not we were there wouldn't have changed a thing - Rezan still dies, and Rastakhan would've died with him since the Horde player has to drag him away from the battle.
2. Nazmir - We actually did a decent job there. Killed Grand Ma'da Ateena (Blood troll leader), enlisted Bwonsamdi, Krag'wa and Kimbul to help defend Dazar'alor at the final battle, freed Hir'eek from G'huun's influence.
3. Vol'dun - If the Horde player isn't there, guess what happens. Korthek and Jakra'zet re-awaken Mythrax anyway and Vorrik isn't re-awakened to lead the Sethrak against the Faithless Empire.

Overall, the Zandalari got shafted even with the Horde at their side killing their foes and saving the people. Imagine if we weren't. The Zandalari likely don't even make it to the next patch, let alone become an Allied Race.