Law of diminishing returns:
In economics, diminishing returns is the decrease in the marginal (incremental) output of a production process as the amount of a single factor of production is incrementally increased, while the amounts of all other factors of production stay constant.
Basically, our OL is already elite, so while Tunsil is an upgrade at RT, the impact of that upgrade would supposedly not be as great as the impact of an upgrade at DT, LB, WR, etc.
So even if Tunsil is graded as a "guaranteed Pro Bowler" and is your 3rd ranked prospect, taking a DT who is graded as a "borderline Pro Bowler" and your 8th ranked prospect would have more of an impact on the overall team.
This has nothing to do with how many plays Tunsil is going to play, whether or not he is an upgrade over Free (he clearly is), it's about the fact that our OL is already elite and doesn't have as much room to improve as our LB corps, the interior of our DL, etc.