Kerfoot already brings useful contributions on offence, but has the added benefit of defensive and multi-position utility which Ryan Strome does not. I fail to see how Strome would be a good "value add" next season when he is a pending UFA, unless you are saying he would re-sign with the Leafs for significantly less than Kerfoot's $3.5 million cap hit.
I would agree that on paper, Ryan Strome brings higher offensive upside, but doesn't have the versatility of Kerfoot. You'll also have to discount Strome's upside in Toronto because he will not be getting a plum assignment next to someone like Panarin. He's more likely to be the 3C and on the 2nd unit PP. I haven't dismissed anything. I've weighed the relevant context. You're just getting things wrong.