When the PB game was in its infancy, the ideal was to have PB values compared for trades, and if there was not too much of a discrepancy, the trade would automatically go through.

Clearly, PB value does not reflect age or potential. It is a counter stat. Adjustments to account for future potential are subjective. Downward adjustments for elderly players, e.g. Julio Franco, are equally subjective. Trade evaluation will always be subjective.