More customization = More variety. If you cut down on options, then (surprise!) your meta shrinks.
"Balance" is subjective. Everyone plays a little differently. Having more options means more playstyles and skills can be recognized and rewarded.
Why would Hunters get their supers more than Titans or Warlocks?
No one wants a perk like Vorpal weapon to completely shut down supers that take 10 minutes to get.
Aim assistance is a big reason why Destiny's gunplay feels so unique and good. I wouldn't mess with Destiny's gunplay.
TTKs are already fairly slow compared to a lot of FPS games. Also, primaries have to be balanced against Special Weapons (which instant-kill) and abilities. If you change the TTKs of primaries, then you destroy game balance in favor of special weapons.
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TTK "compared" to other games is the problem. This isn't CoD.
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Destiny 1 (the original game) used the TTKs we have now. Most seemed to enjoy these values. The original Destiny 2 used significantly slower TTKs, and it was not well-recieved. You may personally prefer the way Destiny 2 launched, but my understanding is that you are a minority