As you've noted, you haven't take bonus attacks from critical hits into account. If you have two attacks and a 5% chance of a critical hit you will get a bonus attack from critical hits a hair under 10% of the time. It's close enough to be 10% and that means your damage will increase by just about 5% from critical hits.
I rarely see the bonus action attack accounted for in discussion of the feat. I'm not sure why. Heck, it's an ability that works with any melee attack! If you combine the chance for extra attack from a critical hit, which is fairly easy to do, and the chance when you drop a creature to zero, which is a bit harder, it pushes GWM much farther ahead than not having the feat.
Try sometime and do the math to see how much damage increase is from the -5/+10 part of the feat and how much is the bonus attack. Yes, it requires assumptions, most notably on how often you drop a creature to 0 hp, but it's worth it.
I rarely see the bonus action attack accounted for in discussion of the feat. I'm not sure why. Heck, it's an ability that works with any melee attack! If you combine the chance for extra attack from a critical hit, which is fairly easy to do, and the chance when you drop a creature to zero, which is a bit harder, it pushes GWM much farther ahead than not having the feat.
Try sometime and do the math to see how much damage increase is from the -5/+10 part of the feat and how much is the bonus attack. Yes, it requires assumptions, most notably on how often you drop a creature to 0 hp, but it's worth it.