We certainly agree in your first sentence but I do think there is a "concept" of pleasure which is used as a placeholder in these discussions where the discussion demands a single word that references the full concept.
I don't think all concepts constitute Platonic ideal forms, do they?
I'll give you that. I suppose we can use pleasure as a word that refers to a group of positive sensations the same way we use dog to talk about a particular kind of animal that is a member of a group of domesticated canines... without going down the Pleasure and Dog ideal form route. We just have to remember we're talking language and not philosophy (although, yeah, the line is fuzzy).