Defensively versus Columbus both guys were basically at the bottom of every category for Leaf forwards for CF%, FF%, SF%, xGF%, SCF% and HDCF%, so any edge either had was basically irrelevant since both were bad. However, I'm not sure the bolded is even true. Robertson had the better FF% and SF%, but Kapanen was better in all the other categories, including the vaunted xGF%. But like I said, in the end neither played the kind of defense that should be seen as a positive in that series.
As for offensively, Robertson was consistently below Kapanen in all the various "For" stats. And that's with Kapanen being lousy.
I just think that penciling in a teenager who didn't even blow things out of the water in his first trial in the NHL to be able to replace or even upgrade on Kapanen's production is asking for trouble. I heard the same thing last off season about Kerfoot replacing Kadri with no issue, and I don't think anyone can still argue that the Leafs didn't downgrade in that situation now.