Orr essentially retired at 26 and is considered top 4.
(Not sure if mcdavid is top 10 yet, but just saying this argument may not Hold up)
Recently, on the History forum, the question came up - why is Bobby Orr ranked in the top four, but McDavid isn't? After all, the gap between them is only about 50 regular season games.
The answer:
1) Orr did things that nobody in NHL history has done (either before or since). He set all-time records. Orr is the only defenseman to lead the NHL in scoring (which he did twice). He holds the record for most Norris trophies all-time. He's the only defenseman in NHL history to win multiple Conn Smythe trophies. And he's one of only two defensemen to win multiple Hart trophies (and the only one since the creation of the Norris trophy). McDavid's trophy case is impressive, and he's starting to move past the Jagr/Crosby/Ovechkin tier (depending on which trophies are included). But he hasn't separated himself the way that Orr did.
2) Orr was an all-time great playoff performer. He was clearly the best player on two Stanley Cup winning teams (he won the Conn Smythe trophy twice). He was probably the best player on another Stanley Cup finalist. Ever when his teams lost, he was usually the best player. McDavid has been excellent in the playoffs the past two years, but we need to see more and/or deeper runs from McDavid before we can compare him to a multiple Conn Smythe winner.
3) Orr was an excellent defensive defenseman. This is supported by statistical evidence (looking at the Bruins' goal differential when he was on vs off the ice - no other player in history had such a big impact on their team). It's also supported by reading accounts of Orr's defensive skill, written during his career. And it's fairly obvious from watching him play. McDavid's critics exaggerate his (supposed) defensive inadequacies, but he's not exactly Patrice Bergeron either.
I agree that Orr and McDavid have played close to the same number of regular season games. But (with no disrespect to McDavid, who appears on track for 5th place all-time) he simply didn't accomplish as much as Bobby Orr in that time.