I love your use of the prompt here! It is an unusual and interesting one :). I was expecting you to drop a mention of a purple sponge into a poem about something else but you actually made the sponge the focus of the poem. I like your continuation of the metaphor in "the world would be watered with words wrung from grandeur dreams." :). The ideas you've picked up on here are interesting- because poetry of course is universal, a lot of people enjoy it and it's often used as a coping mechanism, but what if it was only for the rich?
I think a small change you could make would be to change "than" to "then"; "Than, thoughts would remain captive". I also think you could leave a space before "... And true happiness would never be invented." to give it more impact and leave a greater pause with the ellipsis. But apart from that it's a very good poem, with the rhetorical musings on what if poetry WERE associated with the rich and the effect that would leave.
Lovely, dear, it's sad there was never any voting :(