No. No, you're just wrong on the first bit. Ignorance towards the lore isn't an argument in and of itself. C'thun and Yogg-Saron were weakened Old Gods, N'zoth is meant to be at nearly if not nearly his full power. Different ball game entirely.
And by your logic that N'zoth having a full expansion doesn't make sense, a full patch centered around him doesn't make sense either. He's still in the limelight to some degree and in 8.3 he does that through being right in your face after years of scheming. It could've been an entire expansion of him just fine. In fact, BfA was technically almost an entire expansion with him as a part of it. The problem is that it was just uncommitted. He's arguably an influence in Kul Tiras issues, particularly Stormsong and through the Naga, then directly becomes a part of the storyline in 8.1.5. So he has been looming for most of the expansion.
But the problem is that BfA is nothing but a mishmash of themes, a bunch of costumes it wears because it's self-conscious. It's at the same time a faction war expansion, one that deals with Blood Trolls and their false Old God (should've been the return of Hakkar, really), and some unrelated stuff about the Drust for the Alliance, mainly, which in hindsight I suppose was a set-up for Shadowlands deathy stuff, alongside Bwonsamdi for the Horde. And then it also has Naga and N'zoth. It wears a bunch of skins, some more than others, but all of them relatively poorly, and it burns at least two groups of enthusiasts that I've seen:
1) War fanatics who felt like there was no real point to the entire war and consider it mostly filler garbage because it didn't do anything worthwhile.
2) Old God fans who think the same, and feel that a boss in the making since the Cataclysm and arguably before that is reduced to a "move along, focus your hype on the next expansion," patch.
In some terms, you could even argue that Azshara fans might feel separately gypped, also waiting for years to fight Azshara, only to be tossed at you in one patch with a fairly uninspired zone and raid. (Other than her, is Priscilla the only noteworthy raid boss in her raid?) They probably feel like Azshara got the Archimonde/Kil'jaeden treatment, too. Although it's yet to be seen what her ultimate fate is. But yes, N'zoth and Azshara deserved their own expansion. Possibly even one expansion each, but I feel like if they condensed Argus into Legion out of fear for fatigue, the same is probably logical for Azshara/N'zoth.
As an aside, 8.1.5 had N'zoth looking like he had some potential interesting interactions with our characters in the future, rather than just popping up and turning into generic targeting dummy screaming threatening stuff at you #151432. I feel like they could've delayed his demise by bringing him to the forefront less quickly. Going into the Shadowlands, it could've been an 'enemy of my enemy' deal, where N'zoth isn't unleashed and isn't an immediate risk, or maybe we don't even know we're being manipulated by N'zoth (Old Gods can use avatars), and we just end up taking out the Jailer or whatever big death entity the Old Gods would likely want to see removed from the board.
And then N'zoth makes the push to free himself now that an important player is removed from the board and reveals that we unwittingly doing his bidding from the start, even if it was a case where both the player's and N'zoth's interest best aligned.
Obviously, that's far too late now. They pretty much came out screaming that we were chatting with N'zoth as soon as 8.1.5 so that particular tentacle can't be pushed back inside its box, but there are so much better things they could've done, starting with at least giving him a single open zone that isn't an old zone with purple goo on it.
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You're right. People who claim that N'zoth is the weakest so he's a smaller deal than Yogg-Saron and C'thun were back in the day are just parroting people who are either misinformed or wilfully disingenuous.
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Something needs to be released for at least 6 months before it can be complained about and only then may its existence and lack of better alternative be lamented and held up as some mistake made by Blizzard for years to come.