Argus/Antorus and Broken Shore aside, Legion wasn’t overly heavy on the demons. Let’s look at launch content:
Aszuna - the levelling done with the heaviest demon content. However, they’re contained to one section of the zone, the introduction. They’re dealt with pretty quickly. The majority of the zone is dedicated to the Eye of Azshara storyline.
Stormheim - demons show up very briefly at the end of zone when the Felskorn call in reinforcements. They’re contained to a few quests though, that’s it.
Highmountain - Dominated by the HM Tauren/Drogbar storyline. I know we get the Feltotem but no actual demons themselves. The Feltotem are only brief too.
Val’sharrah - I’m pretty certain there’s only one actual demon NPC’s in the zone? The giant Deeadguard at the temple of Elune.
Suramar - a fair amount at Felsoul Hold as well as some smaller patrols in the city but that’s it.
The main demon content came with the Broken Shore and then later Argus. If Argus was to be an expansion set after Legion then the “demon fatigue” argument would have been flimsy. It would have been similar to saying we’d get “dragon fatigue” after Wrath going into Cataclysm - considering how prominent dragons were in Wrath (3 raids, 4 dungeons, a major part of the majority of zones, the expansions C-plot with Malygos)
I’ve never understood the “orc fatigue” argument from MoP to WoD either. As someone who played both when current, I never felt orc fatigue. In fact, I’m of the opinion Gorgrond should have stayed in its original form.