What does one say?
So the proposition is: One of Orr or Gretzky is going to be unseated by McDavid.
OK. Well. Once that's corrected and the Op resolves to propose: One of Mario Lemieux or Gordie How is going to be unseated by McDavid, we can begin to outline what constitutes greatness and what Lemieux and Howe achieved and the mountains McDavid has to climb in order to be considered.
Perhaps McDavid is the pinnacle of hockey evolutionary skill. But, you have to win everything there is to win to enter the conversation and we can't have different sets of comparisons to suit preference. We can't exclude what each player had to build upon and where they were in their era balanced against their peers.
Like...Wayne Gretzky wasn't simply a product of his time. He was a generation ahead of everybody else to say nothing of Bobby Orr.
Bobby Orr led the league in scoring twice as a defenceman; that's a perennial Norris-winning, two-time Conn Smythe winning defenceman. The measure that some define Mario Lemieux as greatest by in terms of advanced stats, irrespective of totals enters into the quantum realm with Bobby Orr. The "IFs" surrounding his possible career had all things been equal and had he not been injured (or injured when sports medicine was further along) are breathtaking. But as a stand alone, for what he did in the time he did it, I'm not sure how one doesn't loop back - immediately - to McDavid and state the obvious:
In order to unseat Bobby Orr, McDavid has to do what he's doing now...but as a defenceman and then do it as a defenceman who has won the Cup and the Conn Smythe, and do it twice.
And then there's Wayne and Mario and their two Smythes and their Cups and stats, totals and advanced.
And as for Howe, just on goals, if you take his last season with Hartford and those 19 goals and work back to when he left for the WHA and consider it a base average, Gordie Howe retires from the NHL with more goals than Gretzky.
So...Please...Op...Can we reasonably and respectfully call it a contest for fifth all-time?