Right, but what I'm saying is not "when is a poem no longer a poem?" but "when is a poem no longer recognized as a poem at first glance by contemporary eyes?"

(And again, we're talking about art here, not like radioactive isotopes with half-lives and decay, etc.)

No, this is exactly what I'm talking about: potency and decay in relation to art. In this case, I don't think that painting works to talk about poetry, because cave paintings are tangible first and evoke the intangible. Poems on the other hand, are both tangible and intangible. The physical page does not need to exist in order for the poem to exist, though if people are unaware of its existence then it may as well not exist, though in some sense it still does, as long as someone holds it in their mind. This cannot be said of paintings.

Furthermore, while it's possible for the interpretation of the significance of artistic techniques used in painting of respective movements and eras to be lost or misunderstood, that doesn't matter as much, as at the very least, the painting can succeed on a visual level. The same cannot be said of all poems. Yes, one can look at a painting and recognize it as an object created with representational intent, but can the same thing be said of poems? I don't think so.

If we look at the history of language, for a long time it was simple language for accounting for stuff, until one day it began to become more expressive. Somewhere in that time period are the first written poems. But would we recognize them as such? I think it's quite possible that we would not. Of course, this does not mean that such an object could not become a poem again in contemporary eyes.

Either way, the potency of a poem and therefore it's potential and possible impact on a given population are inherently tied to whatever period or era the poem is read in. The potency of a poem cannot remain constant, because people are variable in taste, knowledge and awareness and the million other things that make us human.