Even then, what if one salesperson builds up better longer term relationships (e.g. by not pushing options the customer doesn't really need) and thus their future sales will be better but their initial productivity looks poor?

Or a team player who is good at opening conversations but not closing? (I know, always be closing)

But yes, with a quantifiable and accurate and business-orientated metric, you can measure some form of productivity. It is still hard for most roles in most organisations.