Why are people acting like the Dark Irons will betray the Ironforge Dwarves and leave along with Moira?
The non-Twilight's Hammer Dark Irons seem rather content with Moira especially considering she's been ruling them for a few years by herself, they definitely accept her as their ruler and so far we haven't seen anything that really says otherwise.
They would also accept her son as a ruler too I'd imagine, given how the Dwarves have ultimately united again so there's a good chance there may be a Dwarf High King again even if the Wildhammers leave as he would have rule over the Dark Irons and Bronzebeards, thus having a Dark Iron High King something every Dark Iron probably dreams of happening.
It's also worth bearing in mind who his father was and they definitely accepted him, so if they accepted him and Moira there's no reason for them to oppose Dagran Jr, on top of this he's part of their royalty bearing the name Thaurissan and the royal bloodline.
Also Moira would betray them either. She's made it adamantly clear that she doesn't want conflicts between the Dark Irons and the Bronzebeards so that her son can actually rule them together. So while she perhaps goes about in the wrong manipulative way, she does actually want the two clans to co-exist.