It’s absolutely a good thing and a right thing that Rastakhan die in the new raid and be replaced by his daughter.
TLDR - It’s good for the Horde that Rastakhan is dead, and you shouldn’t be upset about it.
In one of the novels, we see Zandalar under Rastakhan’s rule during the time immediately following MoP. We also get some bits and bobs about him in the Chronicle. This troll is not a good king, and he is not a good person. There are many reasons the Horde should be totally sanguine–even happy–about his removal and defeat and death.
1.) Rastakhan was never a Horde leader, so stop taking that personally right this minute! He was never loyal to the Horde, and he never thought of the Horde as a peer or ally. He only ever thought of them as a tool to be used or manipulated to Zandalari advantage. This is not up for dispute. His brief displays of mild gratitude when a Horde champion goes through trial and tribulation to save his life do not constitute diplomatic or political friendship with the Horde.
2.) Zul is Rastakhan’s fault. The prophet Zul came to Rastakhan over and over as the king of Zandalar to prepare and help the people in the face of the Cataclysm. Rastakhan ignored Zul (as he ignored his daughter in BfA). His dismissal of the threats Zul told him about led to the total destruction of Zandalar’s landmass and wealth. Zul was dismissed, and so he went off to foment war and a renewal of empire under trolls. Vol’Jin rejected Zul’s evil ambitions and worked with the Horde to undo the damage Zul was creating. Rastakhan ignored Zul and even empowered him by looking the other way. Vol’Jin was a horde leader. Rastakhan never was and never would have been.
3.) Rastakhan was lazy, but he was not without the arrogance and delusions of grandeur that are the legacy of Zandalari trolls. At the end of MoP, we see Vol’Jin and Chen Stormstout (and other allies; it’s a good story) fight off the Zandalari forces who invade Pandaria to subjugate it and reawaken the Thunder King in order to reestablish that ancient relationship of Mogu-Zandalari dominance. It was foolish and evil, and Rastakhan instigated it all (including his intention of enslaving the Pandaren–all of them–in the service of the Zandalari and Mogu). If not for Vol’Jin and the Shado-Pan, it would have been utterly ruinous.
4.) In BfA, we see Talanji at her wits’ end. She has done everything she can to alert her father to the betrayal of his council and the trouble in Zandalar from every corner. He won’t listen. He never listens. He is an arrogant, lazy, pampered failure of a king. He needed to die. He deserved it. Instead of leading his people, he spent centuries making deals with loa to skirt his responsibilities and gain power. The Horde should be glad he’s gone and that his daughter, who is fierce and good, will be replacing him.
No amount of “he was nice to us in our quest chains” can make up for literally centuries of being arrogant, callous, and flat-out derelict in his duties as king. Rastakhan artificially extended his life, and yet for all his long years he learned nothing of wisdom, prudence, or humility. His daughter Talanji, however, seems to be made of sterner, higher quality stuff. SHE will be a Horde leader. SHE will be a stalwart ally. Her father was nothing but a relic who could not learn, and he was a liability for anyone associated with him.
For those who don’t care much about the lore and just want to throw their hands up because Sylvanus is evil and we’re still trying to find our footing as Horde characters after the debacle of Hellscream and WoD, the death of Rastakhan is a good thing. You’re gaining the might of the Zandalari people under a new queen who will actually be a worthy ally.
You didn’t want Rastakhan, and you should gleefully mow him down in the new raid.