You're oversimplifying, Dawg. Even if you think that all scouting observations are worthless because they're subjective, Sweeney's age and level are not subjective. A 19-year-old at high-A is very unusual. Reason dictates that he would have produced significantly better numbers in rookie ball or even low-A. Moreover, the assumption that, on average, 19 year olds fill out and become stronger as they age is a medical fact.
If you manage to pull together a large body of data comparing the major-league performance of hitters to their performance as 19 year olds in high A ball, then you might be on to something. Otherwise, it's extremely unfair and illogical to compare Sweeney's numbers to 22 year olds at high A and 19 year olds in rookie ball. You acknowledge that "[y]ou can adjust for any variable such as age, park or league." Please factor these adjustments into your evaluation of Sweeney.