I don't disagree with the principle here, but one thing to keep in mind is that salaries for free agents are increasing at a greater rate than the salary cap.
The NFL minimum salaries were negotiated in 2011 and do not depend on the salary cap. Below is a link:
NFL CBA Series: Minimum Salaries
To Andy's example, a 4-year veteran earned $715K in 2013 and will earn $775K in 2017; that's an 8.4% increase compared to the 35.7% cap increase during that time period. A good portion of the roster earns salaries that compare to the minimum. This in turn causes the inflation for the free agents to be slightly higher. I'd have to look at the actual roster distribution to see, but I wouldn't be surprised if a free agent making $5.5M in 2017 would have been closer to $3.75M in 2013 rather than $4M, but that's just an estimate.
The greater point remains though. In 2013, Belichick surprised everyone by giving Kyle Arrington a $4M deal. But he had played 75% of the snaps. Harmon, while valuable, still only played 50% of the snaps. I'd expect the overall value to the team to be prorated accordingly -- that is, unless Belichick thinks he'll play more than 50% next year.