Every coach wants to see their young prospect players succeed and hold a roster spot. If only because they have low caphits and can provide value far beyond it. So it's ridiculous that people think Sullivan doesn't want a player like Poulin to show he deserves a roster spot, expecially over someone like Reilly.
Coach's recognize that older players with guaranteed roster spots will show inconsistency, because they get complacent with their position on the roster. Yet despite that, they still stick to the teams system and make it difficult for Coach's to punish them for something like athleticism. There's very little a coach can do to change people like that. Players trying to break into the league are held to a higher standard, because they should be fighting every shift to show they deserve to be in the top league. Harkins for example is who he is, he's not going to get better but his effort is consistent most shifts. Poulin will get knocked down harder for mistakes. He has far more room to improve so will get held to a higher standard until he can show consistency with his positioning on the ice and thinking process during meetings.
It may not make sense, but it's important to realize that the difference between the NHL and AHL isn't talent, it's consistency and following the system. There's many players in the AHL that can deke Slavin out of his pants, but then those same players will spend the next shift forcing the same thing again instead of making the safe play that's expected of them. Poulin will get a roster spot if he follows the system, makes safe plays, then adds his game onto that.