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.

Modifying a now trite-but-true saying from the statistical modeling world: “All productivity metrics are wrong, but some are useful.” I imagine there will always be an infinite number of ways an employee could add value that would be inadequately reflected in any finite collection of metrics, but that does not eliminate the usefulness of good metrics. Managerial judgement is always needed to account for exceptions that require accommodation.

Metrics will never be perfect, they are an approximation of the real world. And some things are unknowable. Maybe a customer goes bankrupt 3 months into a project and you never collect the full revenue. One metric shows that a salesperson closed that deal and another metric shows that his revenue per customer isn't good.

A successful team structure will make it difficult to measure individual productivity because team members spend some time on shared team priorities rather than just individual priorities.

I enjoy team work and I push hard against managers focusing on individual performance metrics because it destroys team work.