Ok, you make a good point. Now that you mention it, I am not familiar with too many options at the Tier 3 level that are simultaneously this powerful, reliable, and easy to pull off during gameplay. Further, the Expend option is supposed to be the gravy on the feat, not the core reason for choosing it.
What if the Expend-for-20 option merely confirms a critical threat that you've already managed to achieve? This reintroduces AC as a defense, since you now have to confirm your crit. Further, it's still pretty good because GoW is giving you a reroll on your initial attack roll, which means two chances to threaten a critical on each attack roll. This also would smooth out something I disliked, which was that GoW was better for weapons with a x3 or x4 multiplier. This change would make it equally good for all weapons.
Additional debate that is now largely beside the point but I'm including anyhow because I love to debate things:
I still am unconvinced that critical hit = death. PCs survive crits all the time in games I run, and I am justly famed as a Killer DM.
I'm also not persuaded that a free critical hit per round constitutes a "Tier 1 rocket" ability. Diamond Nightmare Blade is good for a x4 damage multiplier, can be used every other round, and lives at the Tier 3 level. It also requires no feat investment and can be done as early as level 15. A Spirited Charge with a lance has a x3 multiplier and is a Tier 5 trick. That requires only two feats, and can be done as early as level 1. It's a little harder to use every round, but with the investment of one additional feat for Ride-By Attack, a level 2 human fighter can use it pretty consistently. "Damage rockets" can be found at every tier and every level.