But Konecny already proved he's legit TOP liner at that time.
Farabee didn't yet. Let's hope he'll prove it too.
Farabee had a better season than Konecny had before TK signed his extension. Farabee also spent significant time on the first line with Couturier, acquitting himself quite well. Granted, TK had two seasons of upper-40 points production, so maybe less of a risk.
But it's clear they think very highly of Farabee, and at only age 21, entering his 3rd season, and still gaining strength, it seems likely he'll continue to progress and that his 20-year-old season won't be some kind of outlier. Certainly worth the risk to lock him at what should be a reasonable cap hit.
If you wait to sign him, there's a significant chance you end up costing yourself cap space you could have saved. They believe in him, and so they pounced. "Don't get caught watching the paint dry."