MacKinnon is a special talent, I don't think anyone would argue that. At the top of his game he is a sight to see.
But McDavid is still McDavid. The Oilers have absolutely bungled his tenure so far. Bad GM after bad GM, afterthought coaches, poor drafting (no Holland draft picks are currently playing for the Oilers fyi), and legions of shit acquisitions like Jack Campbell, Milan Lucic (traded for James Neal who melted into a puddle after one year), and some extremely questionable extensions (Zack Kassian, Mikko Koskinen, Darnell Nurse to name a few)
McDavid masks a ton of Oiler organizational problems. I'm not confident that MacKinnon could do the same.