If the price to trade for a given player is Gallup...
... and that trade makes us then feel forced to spend a 1st rounder to replace Gallup...
... and even if the rookie WR we draft is as good as Gallup (which is no given at all)...
... then that means you've effectively managed to tread water at the WR position while trading a 1st rounder for the player we got in the Gallup trade. Because keeping Gallup and actually trading the 1st rounder for a veteran DT would accomplish the same result. Except, actually, keeping Gallup and actually just trading the pick for a DT would arguably lead to us being able to fetch a better DT by dangling our 1st rounder.
If you look at it like that, I wouldn't want to effectively trade a 1st rounder to acquire a 30-year old DT who averages 2 sacks per season. I think that's not a wise use of resources at all.