What if it is aesthetics [beginning in the 18th century] that make it possible to have all these inter artistic crossing? So aesthetics asks us to think about all the arts as one category of things, comparing, contrasting, ranking the art forms, etc... Before that you could have a few hints of "ut pictura poesis," and the like, but not a more systematic understanding of everything all together. Aesthetics leads to synesthesia in the arts. [synthetic views taking origin in Wagner, for example]
I think that is why literary music in the 19th century feels different from earlier text settings. I am starting to understand things now in a different light.
It doesn't matter as much what individual philosophers said, but that the debate was an active and one could start asking the right questions. Aesthetic experience is universal, but the philosophical reflection on it is an extra step that brings other things into focus.
It is ridiculous to claim that people did not have aesthetic experience before the word was invented, as Rosen points out. Even today, not everyone uses the word for their own experience, yet they still have experience we would categorize that way. The invention of the term, though, does bring about a different sort of experience on the intellectual level. It is easy to group together visual arts, like painting and sculpture. It is easy to feel that the arts have affinity with one another; but to be able to ask deeper questions leads to deeper developments.