I like the first four lines of the poem and I like the phrase "agestral bed". The rest of the poem tries to present the concept that "nothing lasts" in rather simplistic terms, or should I say, simple language. The use of the word "agestral" amidst the rather common language concentrates the focus on the dome-shaped crown of the leaf (over the raindrop) which holds little relevance in the rest of the poem after its mention. It is in fact the raindrop you want us to focus on as the medium through which you will deliver your point. In the middle of the poem the focus drifts from the raindrop to multiple raindrops when it might be better to continue building the single raindrop up. Basically, I think the word "agrestal", though adding color, does not really fit in with the rest of the poem's tone.
I might suggest that you examine another "truth" of nature that ties into life's philosophy. The word "agrestal" refers to uncultivated plants (like weeds) flourishing on cultivated grounds. An agrarian angle of things growing wild in cultivated fields could lend to an interesting analogy that you can draw from for creatively making a statement.