So yeah, sure the downside of staff is that it is limited to 15m, there are several upsides though.
It is able to apply all that damage to a much larger area as it hits everything between you and 15m in a very wide arc if you sweep properly, whereas the bb only applies its stack in a 4m radius around the bb target. Given that melee elites need to be next to you to actually hurt you the staff is more effective in the meaningful range.
The staff is also applying stagger to everything it hits, which will hold them in place and works well even when they are attacking you, whereas bb’ing in melee range isn’t very safe. The stagger is also preventing them from attacking teammates, which is very beneficial.
That staff demo isn’t using the ult, and so it is possible to do the same thing again immediately and repeatedly. The shredder trick is using the ult to apply half of the stacks, which limits it to a once every ~30 seconds trick because of ult cooldown. Without the ult the only source of more soul blaze for the shredder is more bb, which will mean it falls behind the output of the staff very quickly. You’re probably thinking peril will limit it, but in practice that isn’t the case as the kills in an intense battle will be enough to let you do it indefinitely without ever needing the quell.
The staff doesn’t require any ammo, and so it is able to do that constantly throughout a series of long intense engagements, whereas the shredder eventually runs out of bullets. Indeed because the staff isn’t using the ammo it means those bullets are available to your teammates to help increase their ability to deal damage. Of course this only really matters on hi damnation because the reality is there is ample ammo in any other situation.
I’m not saying that the staff is better than the shredder trick, just that it most certainly isn’t outclassed by it in meaningful combat situations.
There are quite a few ways of using this mechanic in interesting ways, which this video covers pretty well. There are a few more uses not mentioned, like thrust boosting flamer ticks, or slaughterer boosting bleed or burn ticks, but you get the idea.
Re replying. You shifted the goalposts, which is a style of arguing that imo isn’t worth engaging in. You made a claim, I responded directly to it with an example of it being an obviously intended mechanic, then you said it was a bad example and changed what you were claiming.