You're right in a sense, that people tend to overpay for marginal talent in a bubble. You can always look at a cheaper player and go "he's a better deal". But if you keep making that decision, you'll wind up with way too much leftover and not enough talent to compete. Talent's salary is exponential.
Secondly, this draft may be full of D-line talent, but in a similar fashion they might just get overdrafted and you could wind up with nothing.