What's my gig? Not being a hater who twists the lore into a cheap caricature and then try to pass it off as the real thing? That's the difference between my mindset and those of people on the forums. I see something that doesn't make sense, think about the lore, and try to make it fit. Other guys see something that doesn't make sense, don't even try to figure it out, and come on here to complain it doesn't make sense. I see it all the time when people post stories online. People read a chapter, don't get something, and post comments screaming about plot holes and retcons and what have you. Only for it to be explained in the very next chapter. WoW's story is a book series we haven't seen the end of. Hell, it took us a decade to learn more about the vrykul after we first met them.
As for your entire "how the aristocracy resolved its problems" thing, you can't establish a pattern with one incident. For all we know that tal'ashar was the only one in the last 10,000 years, and was only spurred on by the felborne becoming more violent from exposure to fel magic. And as we see from that very storyline you don't even need to do it yourself, you can buy a champion to do it for you. So your point doesn't hold water.
As for literally eating and drinking mana, it can't be that huge of a perk for the average member since they can run out and starve in a matter of weeks. It's not like they're standing by it channeling all the time like the nobles and Elisande were, so I think you're overestimating that.
As for Jaina's record we have all her battles in the RTS, including fighting the Scourge, reclaiming Theramore, and fighting the Legion at Hyjal. And from MoP onward it's been one power up and battle after another, starting with the Staff of Antonidas getting imbued with leftover Aman'thul power. Not sure why you brought up Wrathgate, she wasn't involved there tmk.
And going back to my previous point, Jaina's magic is an outside context problem to Thalyssra. She's a master of the magic the nightborne use, that doesn't make her an expert or even knowledgable about the magic other races use.
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Hit the nail on the head. Any explanation for WHY Jaina won that fight are whittled down to "bad writing durr".