The thing is, though, that performance can be measured, to a degree: if a 'forger' produces an original work that passes as the work of an acknowledged master,

The thing is, though, that performance can be measured, to a degree: if a 'forger' produces an original work that passes as the work of an acknowledged master, then, in purely performance terms, the two artists could be regarded as being at the same level, and if the art world were only interested in performance, it would not care about provenance. Works of art are more than just the objects themselves, however. In particular, works that have expanded the boundaries of what is regarded as art have value as historical artifacts, and the artist who created them reached a higher level of performance, on that particular work (as originality is an important aspect of performance), than those producing works that follow the pattern (including that artist, when producing subsequent works in the same vein.) This point of view would assign considerable value to Duchamp's original 'Fountain', but not much to a replica, which would compare unfavorably to other, original, examples of signed plumbing. If you want a replica of it, there's probably more intrinsic value in making one for yourself than in buying one.