In saying that designers don't listen to components, I think you are thinking, perhaps, about more modern designers, who have a wealth of choices available to them off the rack, or to spec, and have crossover design programs on their computers.
But older speakers (which often are the most likely candidates for recapping), such as ARs, KLHs, Bozaks, and so on, were designed pretty much from the bottom up - I talked with (and bought a Mac MR 67 from) a retired MIT professor who had helped Ed Vilchur optimize the magnetizing of the drivers for the early models, and I remember reading about Henry Kloss with his room full of Waring blenders each with a different formulation of pulp for cones, and his little musical carousel whose tinging he used as a reference for the high end of his speakers. Certainly they listened carefully (and most people in hifi then actually knew what acoustic instruments sounded like, so their listening was informed listening). And they would have been hearing the ESR of caps when tweaking. So we are more likely to hear what they intended if we are aware of its importance.
We can blame them for not anticipating that electrolytic caps would degrade over multiple decades, but no one expected that speakers would be used more than a half a century after they were built.
