I think there is use in looking much further back. If we are looking only 30 years (or even 300) we are not looking back to where the decline started, only at that recent point of the decline. The issue with this to me is setting a bad 'target' to aim for for conservation.

Say 30 years ago the fishery was at 80% depleted, and today it is 95% depleted, then conservation looking only 30 years back could decide to stop (or more likely lose political will) at that 80% mark.