Sure, but that's always the tradeoff. If we know in advance what the data will be used for, we can choose an approach which allows that; although it may prevent

Sure, but that's always the tradeoff. If we know in advance what the data will be used for, we can choose an approach which allows that; although it may prevent subsequent 'fishing expeditions'. Note that GDPR already requires the purpose and scope of personal data collection to be specified in advance, so it's not an unrealistic requirement.