Hard to say without hard evidence. The only ones who’ve said this was the case was Blizzard but I have a hard time believing what they say lately. The problem before with raiders is that they were on a weekly lockout and once they’ve cleared the raid there isn’t anything left to do. Now we have side content that Blizzard could’ve easily grown that could give plenty incentives outside of just creating a power creep.
Never once did I see a PvP player who got their full Conquest and then decided that they’re done with Arenas. Even though they had their full set they continued to do BGs, WPvP or even Arenas to push for the cosmetics that were tied to rating.
Imo, they could utilize M+ as a side activity for the raiders like they already have. Create a theme for each season like lets say Waycrest Manor is the theme. Add new thematic affixes for it and create an Elite set, Tabard, Cloak, Enchant, Mount and title for clearing high keys.
Same thing with Warfronts. Make a niche experience that RTS players can enjoy. And create rewards that you can actually work towards. Rinse and repeat with the next idea.
I’ll never support the idea of tackling someone to the ground and telling them to keep rolling the dice. We don’t need 20 versions of the same item.