As long as passives and skills regarding ice staff are defensive oriented, frost damage will always be associated with tanking rather than damage, unfitting the "Destruction" part of destro staves. At the same time, Inferno is almost always a superior choice unless you play a specific sorc HA build. Here are my proposed changes how to reduce this problem, allowing for more diversity in stick choices. Suggestions here are mostly to turn Frost staff into proper dps weapon, while having an option to retain the defensive powers for all 5 of you that use frost staff for magicka tanking.

Skills
Wall of elements: Inferno wall deals 10% more damage to burning targets, shock wall sets enemies off-balance, frost wall applies minor breach and snare. The inferno obviously deals more damage, even if taking Exploiter CP into account when off-balance can be achieved with many other sources. AoE minor breach is nice but the Scribing system you can put breach into many skills, including very handy AoE Soul Burst pull.

Suggestion:
-Inferno wall deals 10% more damage to burning targets - this is fine.
-Shock wall makes enemies standing in the wall take 500% more damage from Concussed and sets them off-balance (the number is out of ass, so I trust devs to give proper value here)
-Frost wall have the minor breach removed (do not worry, it will return), instead it will cause enemies to take 15% more Frost damage if they also are affected by Minor Brittle. Snare stays. (again, needs a proper value). Frost Safeguard shield also stays with minor tweak (see Tri-focus)

Destructive Clench: while Frost Clench is tanking oriented, I think the morph is fine because of the major maim debuff. Clench morph is more of utility rather than damage.
Destructive Reach: I'd buff Fire and Shock Reach closer to the level of Frost Reach to give a chance to have other elemental spammables but its not necessary.

Impulse: fire impulse deals execute damage, shock impulse scales of amount of enemies, frost impulse gives minor protection and does nothing more. Clearly Frost Impulse needs something more than minor protection, that in itself is very common buff.

Suggestion:
-Fire impulse deals extra fire damage that has execute scaling, this is fine but needs to have numerical value increased to be worthwhile the magicka cost.
-Shock impulse deals extra damage based on amount of enemies hit, this is fine but need just a little bit more aggressive scaling
-Frost impulse guarantees Chilled status effect and increases its damage. Applies Minor Breach.

Elemental Rage: Fire rage deals more damage, Shock rage lasts longer, Frost rage immobilizes. Again, frost version is CC oriented, unfit for destruction ultimate.

Suggestion:
-Fiery rage deals more damage - fine
-Thunderous rage lasts longer - need extra 1-2s to be more equal to Fiery rage damage increase
-Icy rage no longer immobilizes, instead it either apply Major Brittle for the duration or has increased crit chance.

Passives
Tri-focus: inferno heavy applies a dot, shock heavy deals aoe, frost heavy gives a damage shield. Frost staff causes block to cost magicka instead of stamina. Fire and shock version adds damage while frost is not. Block cost conversion is more annoyance that it is useful.

Suggestion:
-Inferno heavy applies a dot - fine, but let this ability to trigger effects like on damage over time or on weapon damage, it currently does not.
-Shock heavy does an AoE - fine
-Frost heavy gives a damage shields, while damage shield holds or Frost Safeguard absorbs a projectile, you pulse with frost damage in 5m around you (like Iceheart). Block cost conversion removed, and instead moved to blue Bulwark CP node. Why I didn't remove the damage shield bit? To have it still synergize with Destruction Expert passive that gives magicka when destro sourced damage shield is hit.

Ancient Knowledge: Inferno increases dot damage by 12%, Shock increases direct and channel damage by 12%, Frost decreases block cost and increases block mitigation. Again, Frost version is less "destructive" than the other two.

Suggestion:
-Inferno increases dot damage by 12%
-Shock increases direct and channel damage by 12%
-Frost increases critical chance by 10%. Block cost and mitigation moved to red Fortification node.