The completionist logic is irrational, hopefully you can at least yourself realize it. You're putting an artificial value on 100% ownership.
Just because completionism is irrational doesn't mean it's not real. There are people, believe it or not, that derive satisfaction from having 100% of something. More importantly, having 99% of something is like having a hangnail. You could ignore it, but it's going to bug you endlessly. Until my brain stops sending me chemicals that encourage me to be a completionist, I'm going to be one to some extent.
One of the great things about Dominion is that the collector is rewarded, instead of punished, for his completionism. There's never been a Dominion product that I bought just to have it. I enjoy playing with each of the cards. They're all good products that are worth having in their own right.
So how about this, popsofctown. I'll stop being a completionist if you stop eating your favorite food. I mean, you're just putting an artificial value on the food. Your brain may be sending you signals that it's is delicious, but you can overcome that. I believe in you!