Both lines are equally important. You have to win the LOS to win the game. You can't do $hit offensively--run or pass-- unless you have a good, solid O-line. You could have Bo Jackson in your backfield and Peyton Manning at QB, if your O line is no good, neither are they. Likewise, your defense can do so much more if you have a strong D line that can stuff the run and pressure the passer without requiring a ton of back end and outside blitzes.
I would agree with you to the extent that your post suggests that great O linemen can be developed, but great D linemen are born--they have to be more naturally talented athletically than the O line guys. The better programs have a strategy for bringing in and developing O line players--some of those players aren't even O linemen coming out of HS. They look for big, athletic guys with good hands and feet who have the kind of frame that can add a lot of weight with strength training and diet. Brian O'Neill is a good example. But on the D line side, unless you have the natural explosive strength and athleticism like an AD or Suh, there's only so much you can do to improve technique-wise.
A highly successful Power 5 head coach that I know once told me over beers that in recruiting he and his staff emphasized watching OL prospects play basketball, which told him more about their footwork, agility, and coordination. Same with wrestling. He has put a couple of his lightly recruited OLs in the first round of the NFL draft, one he recruited as a 2-star 6'6, 225 pound HS tight end and another who came in unrated at 6'7, 245. I think O'Neill will end up being one of those guys, an earlier-round draft pick. Very impressive that he could switch from TE to OT as a RS freshman and end up starting a whole season.