Why is the game going down every expasion?

[ovIm]

Not only was it a utility skill, it was also important for gauge and more importantly GCD manipulation in Stormblood.

Right, I completely forgot about that aspect as well. Damn, Stormblood really has been quite a while ago...
Thank you though, you are absolutely right - gauge management used to be kinda important for a WAR back then.

Quick doublecheck with the WaybackMachine tells us that, post 4.2 rework, Onslaught was 100p for 20 Gauge, and Fell Cleave was 520 for 50 Gauge (or 104p for 10 Gauge). So, by simply reducing the Gauge cost on Onslaught to 10 instead of 20, it'd make the comparison '104p per 10 Gauge spent on FC, versus 100p per 10 Gauge spent on Onslaught'.

Onslaught is an oGCD though. 100 potency oGCD is worth more than 100 potency GCD.
Generally, constantly pushing Onslaught on cooldown did hurt your damage iirc, so you wanted to be smart with its usage.

[Absurdity]

Onslaught is an oGCD though. 100 potency oGCD is worth more than 100 potency GCD.
Generally, constantly pushing Onslaught on cooldown did hurt your damage iirc, so you wanted to be smart with its usage.

I probably should've touched on that but yes, this.
It was an oGCD so you had to include the GCD you gained from Onslaught into the calculation, since you wouldn't have had that GCD if you pressed Fell Cleave instead.
Now this could be as low as an extra Heavy Swing in which case it probably wouldn't be worth it... or it could be as high as a Storm's Path and the extra 20 gauge from it.

This obviously made it a lot harder to pick the "correct" choice compared to "I have 50 gauge I press Fell Cleave" but it also made your choices more interesting.