I think it's more complicated than that. Suppose you draft guys good enough not only to retain with a second contract, but also good enough that the rest of the league wants to woo them away from you...
So the premise is at best a simplification, perhaps a proxy for something more subtle. Yes, it is clearly correlated with success. But correlation and causation can be confused. Could it be that draft picks are more likely to resign with teams that are successful, not dysfunctional teams throwing money at a problem they don't understand?
Also, I'm confused by your use of the word "value". Just what do you mean? Is it the value in contributions on the field? Is it the value of cheap contracts leaving more cap space for others? What is this "value" of which you speak?
You have a point, but knowing who to resign and who to let go makes a huge difference. Vereen third down back? Sure, he was a 2nd round pick, pretty high, but...how many small quick guys are out there any time? Compared to big athletic linemen, lots. Turns out they found a phenom, but they would have filled the position similarly somehow.
Now, linemen as talented as Seymour and Wilfork? They are very hard to find and if you do, there's usually character or other concerns. Negotiations got acrimonius too, but they locked them up. Vollmer too, though not as difficult.
They definitely wanted Welker, but when his agent started being a jerk, on the phone to Amendola...and Amendola wasn't even the replacement. How many quick receivers have they picked up? Keshawn Martin looks pretty good too.
Pats want to extend players who love the game and are good team players but, realistically, if you're going to challenge on a contract, you better be irreplaceable.
It's interesting that two of the worst second round picks (one is deceased, no disrespect) were faux replacements for Wilfork and Seymour and they apparently fooled no one, especially their agents. "There's only so many huge, athletic people who can play football" is a paraphrase BB has quoted from a source i can't remember.