The problem isn’t with the rune word Obsession. The problem is with Staves in general are disadvantaged compared to Shield and a one handed item (like HotO or a rare Orb).
Obsession actually does a fairly good job of dealing with that by adding massive Resistances, and the Chance to Cast Weaken. But it’s still not quite enough.
I have a thread on changes needed to base weapons over in the General thread, including Staves. Here is my proposal for Staves:
Staves are of course targeted primarily at Casters, and suffer from two problems. The first is that a one handed weapon and shield can provide more modifiers than a single two handed weapon. This is particularly important to Casters as the two most important mods for them are +skills and Faster Cast Rate. Not much can be done about this except to buff staff stats to be competitive, similar to the new Obsession runeword.
However, there is also the second problem that shields provide block, and two handed weapons do not. This does not simulate well the fact that one of the ways a staff was historically used was to parry attacks.
Essentially, we need a “new” mechanic that allows a staff wielder to negate a melee attack either when attacking, or when standing still. I put new in quotes because in fact this mechanic already exists in Diablo II. It’s called Dodge, a skill on the Amazon.
I propose cloning that code, calling it “Parry” and give a % chance to trigger when being attacked when wielding a weapon with that attribute. For Staves I’d suggest something like 20% to 30% for the Normal Staves, 30% to 35% for the Exceptional Staves, and 35% to 40% on the Elite Staves.
I go into more detail in the thread about how this would interact with Amazon using Dodge, etc. But the important part for Sorceresses is that quoted above.
If you implemented that change, suddenly you could get an Obsession base that also has up to 40% chance to “Parry” as well, and that really would make Obssession BIS for a Nova Sorceress. It would also continue the aspect of a perfect Obsession base being a really rare find, but at the same time allowing for some less than perfect bases to still be better than HotO + Spirit.
For Changes to Obsession itself? Maybe, maybe increase the Faster Cast to 75%. That would allow reaching the last break point with only one Faster Cast ring and make its Faster Cast rate equal to a HotO + Spirit.