Best way to look at it is, 10 isn't very many, is it?
If you're just looking at the points, you can get Gretzky (obviously) and Jagr over him (that's 2). Most casual fans are aware of how good Lemieux and Orr were (that's 4). And you don't have to dig too deep to understand the many advantages Howe has, with 5 Hart trophies, more goals, nearly as many points in a lower-scoring era, and more post-season all-star nods than Messier has all star games (that's 5).
By now, you'd be wondering why you've locked in 4 forwards, one defenseman and no goalies, and you'd have to think that it's time for at least a few of Harvey, Bourque, Shore, Lidstrom, Hasek, Roy, Plante Sawchuk or Brodeur (pick two and that's 7).
You'd have to recognize that by now Crosby has the same number of Harts, an extra Smythe, and two scoring titles Messier doesn't have, at least consider what make Jean Beliveau and Rocket Richard the legends they are, despite it being a little harder to accomplishment-count with those guys. So if you're taking all 3 of those guys, that's 10 right there.
But if that's not enough, you might ask yourself whether Bobby Hull's 7 goal titles before he turned 28 was a bigger accomplishment than all those points Messier scored, and whether Ovechkin being right behind at 6 goal titles by age 30 stacks up as well.
You'd want to look into Stan Mikita's scoring dominance in the 60s, and Guy Lafleur's short but blinding peak.
You'd have to be at least curious about what made Howie Morenz special, even if you ultimately decide players from before WWII aren't for you.
Anyway, I've named 22 players you could all get above Messier pretty easily, depending on how highly you value their best attributes, and that's without getting into a few others worth talking about, including Bobby Clarke, Denis Potvin, Phil Esposito, Red Kelly, or any of the Soviets. You don't have to rank all those guys over Messier. You might even think he's better than the majority of them. But you can get to 10 pretty easily if you open the field beyond guys who put up a lot of points in the 80s and 90s.