Durz, uhh, they didn't nerf staff. They nerfed Chaos Storm (the only non-target restricted AoE Mesmers have), and pretty much all Phantasms (except Berserker, which kills itself often enough?).
Phantasm builds are no longer allowed to have momentum, to build up steam. Except they weren't buffed and limited to one, they still shatter or die in a single hit, but are expected to carry how much of our offensive pressure, but now they're pushing it as if they want it to take a minute or more before a Mesmer can reach their full damage potential (assuming some Phantasms live longer than one attack cycle).
Chaos Storm as a Point Cap is mediocre damage, but moderate levels of mitigating conditions at work. Staff doesn't have the knockback of Greatsword, or any of the deniability of a Guardian (who literally can find how many ways to say GTFO), area coverage like a well necro (with actual condition damage).
If people stand in Chaos Storm, they take damage and weakness and whatever else. If they move off to the side they can still cap a point (and are even free to decide which dorection they go, unlike Guardians), and keep doing damage. Shucks, you can't reach the Mesmer dancing in the middle? Pull out your ranged weapon and smack her hard. Same with a Guardian, a mobile ranger, whatever. Chaos Storm, the ONE aoe the class has that's not target restricted isn't exclusive in this.
They have no idea what they even want Phantasms to do. Most of the traits don't even fucking work (Fury, Haste, Retal, and the lags from Regen except on Duellist...?). Stop using your "I can kill bads so easily, the class needs nerfs" as justification for these nerfs (which only make it harder on Mesmers) but do nothing to fix the issues people are having with the class or even why people are having trouble against Mesmers in the first place.