Rantanen and Landeskog were Mackinnon's wings, Kadri barely had time with them and definitely not enough to really have a noticeable effect on his production. Kadri spent most of his 5v5 time flanked by Nuke and Burakovsky. Good players, sure, but neither had ever put up high end numbers before last season. And even then, Kadri finished with only 2 fewer 5v5 points than MacKinnon. I think it's completely fair for someone to think no forwards on our team could have replicated that in the same position, other than maybe Barzal.
That said, I do think he will miss playing with Makar at 5v5, not to mention on Colorado's PP, so I don't think he will replicate the same numbers if he ends up here. But I would still expect him to easily be one of our best forwards offensively, and wouldn't even surprised if he were our best. At least until he slows down.