Anytime you double up on a d-lineman you're creating another hole somewhere else. If you double up on a d-lineman consistently as part of your overall scheme, you're committing suicide.
Turning a TE into a strict O-Lineman just frees up a backer or safety, which takes away your intermediate pass game and forces you to run. Again, it limits your scheme which P5 defensive coordinators would eat you alive doing that. In Iowa's case it's even worse because teams are putting 8 guys in the box and blitzing half of them. Adding a TE to help block is like trying to stop Niagra Falls with some sandbags and duct tape.
What it boils down to is at lower levels of football you can compromise positions to help out weak areas. Like in high school you'd probably be able to get away with giving up a TE, etc., but at the B1G level there's just no way to be successful without being solid everywhere.