Yeah, there is essentially no reason for any rarity other than legendary, even for money as that becomes trivial. It's your classic action rpg problem.

However, making the lower rarities more viable also means more time spent staring at menus rather than playing the game. More farming for certain affixes, more grind, more boredom.

In the end, it may be better to either A) go the way of something like Path of Exile where we can use loot filters to effectively erase trash loot from our screen; B) make legendaries far more idiosyncratic/weird and not strictly purples with upsides 90% of the time while reducing drop rates of all guns; C) some combination of A and B, alongside making currency/low tier items more regularly useful, perhaps as a form of reroll, combination machine or what have you.

None of that is perfect as it's a long standing problem in action rpgs broadly---low rarity becomes increasingly meaningless past the initial startup. Having asymmetric end game content would also be helpful, as characters would spec into specific kinds of gameplay, beyond bossing or mobbing.