All guns are usable, you're using Red Death, lots of people use SUROS, I like Thorn and Up For Anything.
One gun being flat out better than another is part of all these games. Otherwise you could have kept your first gun from PVE and ran with it the whole time.
Even Halo, a lot fewer people used the battle rifle once the DMR was available because it did more damage per shot. It didn't make the BR useless.
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Know what else happens in halo? All guns are viable. In destiny most exotics and basically every legendary is outclassed by the top tier guns quite heavily
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So again your problem is with the foundation of the game. If the Vex was the same as Suros what's the incentive to try and get it?
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Pretty much every gun is viable. We all get killed by the "better weapons" but you just need to suck it up and learn the maps. Knowing where your enemy is coming from exceeds the need for the weapon you think is a beast. In a game where grenades, heavy weapons and supers give so many kills, there is no reason to whine about your primary gun.