Looking at this question from a slightly different perspective, does anyone have any objective criteria which would inform us at which point of scratch/ internal dust/ other, that the performance of e.g. a Leica lens looses its erformance advantage/ characteristics. such that you would be better either replacing it whether with a new cheaper alternative or minty Leica? The problem being that we tend only to have one model of a given focal length hence unable to make the comparisons for ourselves.
Or does it matter if we are happy with the results as they appear before us?!!!
Peter (no scratches)