The way we measure educational attainment across cultures is by PISA scores.

The whole basis of 'US is doing worse than OECD or Greece' or whatever is based on PISA scores.

It's very crude, but it's what we have.

My concern is that without some form of broad standardized testing, we won't have proper metrics to measure the success of programs and/or teachers against.

I agree (though I challenge the utility of current standardized testing to fulfill that job-to-be-done). My position is that we are already paying tens of thousands of teachers to assess the ability level of their students everyday while maintaining a secondary assessment industry that is able to exist because teacher's assessments are not able to be trusted. If we can make teachers' assessments trustable (for ranking/sorting/certification of skill & knowledge) then we won't need the very expensive secondary assessment industry to nearly the same degree. Likewise, many of the negative externalities of standardized testing could be mitigated by ubiquitously assessing student ability rather than relying on traditional "snapshots" as is the status quo.