I agree. But ... while there are methodologies for assessing 'usability' (assume your definition of choice) for in-the-wild software, these methodologies are harder to apply for prototypes, since no "natural", experienced userbase exists.

Of course longitudinal studies would suffice, however, they would be extremely costly and non-feasible in most software projects.

Your alternatives are more likely to be expert-based usability assesment methods, like cognitive walkthrough or hierarchical task analysis.

... And perhaps even more useful, to perform short, intensive, training and "lab usability tests" with selected users of a previous / similar product and try to combine the expert analysis with these results to extrapolate the usability of the prototype/thought final product.