I don't think you understand hockey.
When you're drafting top five in the NHL Entry Draft, what you want is to walk away with the a Dale Hawerchuk, Steve Yzerman, John Tavares, Ron Francis, Sidney Crosby, Patrick Kane etc. who can hit the ground running from day one and be the cornerstone of your team, showing improvement every year on the road to stardom and Stanley Cup contention. It's as simple as that.
Barring those ideal results, teams will pick a player with some ratio of potential upside and NHL readiness. You pick a guy like Marner who is long on potential but short on maturity, you have to wait. You pick a guy like Luke Schenn who is physically mature but has no upside, then he makes the NHL early but has no room for growth.
But to suggest that an NHL team doesn't want an NHL ready impact player and would rather develop them slowly is a fundamental misunderstanding of what makes this league work.