I understand your point. Flip might use that as a reson to prefer Okafor to Towns, but no one with any sort of intelligence should come to that same conclusion.
The rookie first round deal promises the player for a minimum of 5 years. And historically teams retain those rookies for at least their first contract after since at this point 23 year olds are more concerned with their financial future than anything else. There are little to no exceptions of this.
You're talking about essentially locking a player up for 7+ years. Even under the disaster scenario of a player taking his 5th year rookie deal qualifying offer and leaving after, it's still not a concern, as good players are rookie contracts are the most valuable thing in the entire league.