Heres the ultimate problem with this situation, and I understand this is a dev issue, but bear with me - if I have to create loot that is exclusive to the activity or challenge, this means I cannot create challenging activities in a quick manner because I am always going to be depending on art/design/programming to implement that loot & to create it and ensure its compelling.
Diablo 3 didn't have an update to its gameplay that went beyond adding new challenges that weren't just remixed preexisting content, and it kept it afloat for nearly 5 years. The amount of new art added was very limited, both in terms of weapons/armor or levels and enemies. If I am going to make armor/weapons exclusive to the challenging activity it takes to get them, then I have to make wholly new content every time, and that is ultimately very unlikely given the on-going MTX structure of the game (no paid expansions/everything is cosmetics). A lot of their ongoing funds is going to go towards generating a lot of cutscenes & story events, which means they will probably have to update the endgame loot table, rather than creating new layers to it.
With this in turn means is that the best short-term solution towards maintaining the feeling of rewards for the most challenging difficulty shouldn't be the reward themselves, but the volume of rewards that is produced. This is exactly what Diablo 3 did, and this game's gameplay & incentive design mirrors it more closely than any other loot-driven game.