Why is the game going down every expasion?

[Absurdity]

I was actually counting the minutes until someone would say that lol.

20 Gauge OS was amazing design because it discouraged spamming it, the opportunity cost (DPS loss when not used to keep uptime) meant it was an actual situational utility skill more useful for its intended design (closing gaps) than an extra bland and boring damage button you try to keep for your burst window.
Whether it was used often in the fights back then or not is irrelevant, the skills design was much smarter then than it is now. That being said, good for you that it was not needed on your strats, I got my mileage out of it and it was fun to use when I had to because for some reason I did sometimes end up in Narnia and had to keep my GCD rolling.

(ETA - yes i know that you still get mileage out of the current iteration of the skill by using it to close gaps and keep the GCD rolling - thats not the point I am making here)

Not only was it a utility skill, it was also important for gauge and more importantly GCD manipulation in Stormblood. Depending on downtime and boss invuln phases you could spend the gauge on Onslaught (since it was roughly equal to Fell Cleave on potency per gauge spent) instead of Fell Cleave to end a phase on a combo finisher instead of just sitting there after a Maim and having your combo drop while you couldn't attack.

Anyone saying Onslaught was just a button you didn't want to press outside of IR simply didn't understand it's uses properly.

The same goes for Inner Release. I don't know why people praise the current 3 stack system when sure, it fixed latency issues, but it also removed any interaction between your kit and the guaranteed direct crit for 10 seconds. No longer can you aplify Onslaught or Upheavel, or selfhealing through (at the time) Inner Beast and later Nascent Flash, or counter damage from Vengeance.
But that's ultimately what all the warrior changes over the years boil down to and why I also consider it to have gotten worse, the removal of small optimizations that were never required of the casual player but brought depth to otherwise incredibly basic gameplay.

I also find it incredibly funny when people call Warrior's gameplay "just spam Fell Cleave" when every change since 4.1 has just continued devalue Fell Cleave itself as an ability.