Using first 6 years WAR might be a cleaner way to look at it.
With an age 30 cutoff, you'll get guys who break in early and accumulate more than 6 years, and you'll get guys who break in later and have less than 6 MLB years before age 30. It might all even out, but it might not. Like you say, the better prospects might be getting 7, 8 years on average here, and the worse prospects might be getting just 5 years. So that would change the magnitude of the relationship here - making the more valuable picks even more valuable, and the less valuable picks even less so.