Agreed about the Janitor and sort of Sargeras (he just showed up at the end of Legion, said "No", and left). Disagreed about N'Zoth.
N'Zoth's build-up was done extremely well, which is why Blizzard got such a huge backlash with that travesty collectively known as the "Ny'alotha raid".
People were talking and speculating about N'Zoth since like 2010, since he was the only remaining Old God (that we knew of, then Chronicles confirmed this) and he was the orchestrator of Cataclysm. He was indeed the master behind Deathwing, who infused him with power and ordered him to invade the mortal world. The main villains of the expansion (Elementals, Naga, Twilight's Hammer, Faceless Ones) were all taking orders from him. He was the secret puppet master controlling everyone and everything from the shadows, but unlike the Janitor, this was actually good, because we knew since Classic that these ancient creatures known as "Old Gods" existed and that they were behind the corruption of Neltharion, Azshara, etc. etc. etc.
So actually N'Zoth did have 10 years of build-up. Cataclysm-End of BfA was almost precisely 10 years. And I would say that N'Zoth was EVEN MORE built-up BEFORE the Chronicles were released, since at that point it was stated that even Sargeras would beg for a quick death if he had to face the Old Gods. The Old Gods were the ultimate evil of the WC universe, before the Chronicles retconned them into minions of the real ultimate evil, the so-called Void Lords.
Meanwhile in BfA literally no one knew who the Janitor was, because he was made up. Which is why no one will care about the Janitor once he dies.

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