You are still woefully short of understanding what it is measuring....on a conceptual level.
It's linear because each play is judged relative to what an average fielder would do with that opportunity. Let's suppose that on the first play and last play an OF has during a season, it is the exact same type of hit ball, hit by the same hitter, same azimuth, trajectory, velocity, etc, and let's suppose the fielder makes the play each time. You want to limit the amount of credit he is given on the last ball because you think subsequent credit should be harder to attain than initial credit. Do you not see the error here? It's the same play, it's still being judged relative to average, so it should receive the same credit. Thus, it is linear (on a conceptual level).
I realize why you think this should be the case. You think there should be a limit to the amount of impact an OF can have over the course of a season. Realize this, the limits established by your board game are based on trends....averages. Averages are composed of individual data points that are both above and below the mean, with outliers in each direction. You do not prohibit an outlier from occurring, because they are real. If a fielder performs to an outlier level, he should get credit for that, and be recognized for greatness or abject failure depending on the direct of the outlier.
To the rest, you are suggesting an application of a second adjustment. The adjustment scale has already been determined. You don't need to, and more importantly, shouldn't, adjust the data twice. You compile, then adjust. Not adjust, compile, and adjust again. If you use the data and adjustment for position right, then you can compare two fielders for overall impact.