The idea of perfect or near perfect identity between the translation and the original is a metaphysical ideal far more difficult than the demand that the translation be as good as the original. Logically, as Borges has shown, it is not impossible for the translation to be as good, verbally speaking, as the original. It is difficult: say the poet is Borges and I am the translator. Since I am inferior to Borges as a poet, then I won't be able to match him or, even less, surpass him. It is easy to see, though, that this is a contingent fact.
But the idea of matching a poem in all its aspects, and making it virtually identical, a near-perfect simulacrum, is not a difficulty but more like an impossibility. Doesn't translation involve change by its very nature?
Translators are always talking about sacrificing one thing for another, or balancing, compromising. The original poem (as we view it at least) is completely uncompromising. It is what it is. The mentality of the translator, horse trading some meter for some literal meaning, or some nuance for some comprehensibility, is completely different.