Once you start claiming that the arts peaked at some point, millions of other informed, erudite people are going to disagree and set it at some other point or not at all. You say the end of the 19th century was the start of a decline? That was before jazz was a thing, and that’s a genre that plenty would defend as high art. It was before plenty of novelists, poets, painters and playwrights that have both popular appeal and have entered the serious canon. Hell, it was before writing fiction even began in some languages, because for whatever reason (such as censorship by a dominating power) speakers of those languages were unable to do so before the early 20th century.
Moreover, claiming such a peak followed by a decline, and drawing comparisons with Rome, is so often linked to a reactionary politics that even many conservatives feel is extremist, your interlocutors are going to be suspicious of your motives.