I see your argument in theory. No matter what they do, min/maxers will find the best weapon/loudout/whatever, and that will be what everyone is using. In concept, yes, you're right.

But I think that's conveniently ignoring there are subjective assessments we can make of these weapons as to whether they are too powerful that are completely valid and should be open to balancing. As a Blinking, Quickdrawing, Lucky Raspberry wearing Bladedancer rocking TLW/Sniper or Thorn/Shotty player, I completely acknowledge, that loadout is overpowered. They can and should nerf those weapons. Yes, once those weapons are nerfed, other weapons will most likely take their place. But those replacement weapons may not be as strong. This is in fact the whole point of OP's post which I think you're just ignoring. The power creep is already too high with TLW/Thorn in PvP so they're bringing it down. Even if Hopscoth Pilgrim or Red Death just end up replacing it, it will be a net decrease in the power creep in Crucible leading to longer TTKs and less insta-deaths which is what Bungie and the majority of the fanbase want.

This same logic applies to PvE. Gjallerhorn and Ice Breaker is too strong. Black Hammer trivializes certain fights in ways it wasn't meant to. People who can't admit this are delusional. Those weapons need to be brought down in power. Sure, maybe they become noticably weaker and something else like the Truth is now considered the best exotic Heavy. So now everyone is rocking Truth instead of Ghorn. I guess sure, that doesn't solve the problem of weapon diversity, but it does solve the problem that OP was talking about of Power Creep.

And quite frankly, absolutely nothing will solve the problem of weapon diversity because as long as there are any differences in weapon balance, no matter how minor, min/maxers will always gravitate to the best weapons. However bringing the outlier weapons that are so much stronger than the rest at least mitigates it. Right now, I almost never use any exotic right now because Ghorn is just so much better than everything else. However it after the weapon balance patch it's now not so much better, or there's another better weapon, but the gap between it and the rest isn't so large, I am much much more likely to play around and experiment with different weapons because I won't be experiencing such a huge loss in efficiency as currently is. If the difference between Ghorn and the next best weapon is 50% (just making up fake arbitrary numbers, but it's just an example) then of course I won't use anything else. But if after the nerf Truth is the best rocket launcher now, and it's only 10% better than all the other choices out there, then I'm much more inclined to try out other stuff that I haven't before.