Your ignoring the fact that iBerserker almost ALWAYS dies before it attacks or right after it's first attack. Mirror blade. Okay, it gives vulnerability or might but I'm doing that constantly with staff 1, albeit fewer stacks but the constant upkeep makes up for it. And I can't believe you're defending mind stab, sword auto attack does the same thing... And of course mesmers dish out damage from various sources: conditions where staff reigns supreme, reflect, phantasms (ranged phantasms have better sustained damage than melee phantasms), and shatters.
I'm not saying GS is terrible but it is outclassed by mh sword and pistol/focus and staff. PvE enemies very rarely sit at 1200 range which is where most of GS damage comes from.
First of all, I've got a feeling that you're talking about solo PvE in open world, because they surely can sit at 1200 range if you choose to in dungeons and fractals... But for solo PvE in open world, you cast #4 to open a fight, followed by a mirror blade, shatter right away, and the mob should be dead anyways... o_O
And iBerserker doesn't "die in one attack" in dungeons, if the boss ignores it it ignores it. If the boss really likes killing phantasms he'll chase them down no matter what it is. The advantage of a ranged phantasm is that aoe won't hit them as easily, but melee phantasms surely have their place. For aoe in dungeons, iBerserker will be able to get off the first attack before dying --- And that's all you're using it for anyways, you'd 4 or 3 illusion shatter (with mirror image) right after anyways. Over 80% percent of time in any explorable dungeon/fractal, my iBerserker lives as long as I've planned for it to live. This is not to say they never die, but that there's no "surprises" : it is quite easy to predict how many attacks your phantasms can do before they'll likely get killed once you know the contents front to back.
Again, greatsword is the highest dps weapon for a mesmer... Sword and Staff simply do not hit nearly as hard, even considering all the conditions/boons that they apply. This can be VERY easily shown with math. Sword can come close (again, still below GS, but close) if you stay in melee all the time and keep slashing without interuption, but that's simply not possible, you'd lose dps from any dodge, which WILL happen if you want to live in melee range of dungeon mobs/boss.
The point of mindstab is not "what it does", it is "the damage it does". Same with mirror blade. It should also be noted that, if you're going for condition specs, stacking precision, investing into dueling tree to get shaper image, and using Greatsword (greatsword clones hits fast, giving highest chance to proc bleed) is higher damage than staff builds past a certain precision threshold. So the reason to go for staff is not weapon damage itself, but rather iWarlock. And perhaps phase retreat + chaos storm as "added utility". The reason to go for sword is not damage itself either, it is good damage, but the sword shines because of Blurred frenzy 2 second of immunity on a 8 sec CD. If you're a glass cannon aimed at doing maximum damage however, greatsword should be either constantly on one of your weapon sets, OR sitting in your bag ready to be swapped in (because some people just prefer other weapons, and that is fine. But greatsword has its niche --- Damage).