A giant, dominating LT is a sky-high positional value. His post-draft injury is moot. If the Saints get a 4-year starting LT on a rookie contract after missing the first year entirely due to injuries and redshirt season to acclimate him, it would be opposite of a "total waste" -- every team in the NFL would take that value.
The trade is totally impossible to grade as far as an outcome standpoint at this time. If Penning is a good starting LT, no one can look back and say it was a terrible move because it's no longer a hypothetical.
ETA: A good starting LT is around $13M-$20M per season. At four seasons we're talking $50-$80M. Regardless of the sheet trade value at time (which I agree was sheet), the Saints picked a super high ceiling prospect at a super high value position that's also a position of need. I can understand this in context because a successful Penning is absolutely worth that draft trade value and more.