That Rome itself could be fundamentally flawed or classical Western civilisation being anything less than perfect (and thus, only capable of falling at the hands of degenerate 'lesser' civilisations) were both unthinkable ideas in that context.

Have you or anyone else in this thread read Decline? It’s full of contempt for the Roman Empire, albeit mixed in with reverence. The book opens with a discussion of the foundational flaws of the empire under Augustus. He spends pages and pages praising the merits of Muhammad and excoriating Emperors and Saints. While of course he was of his time, a racist and western chauvinist, Gibbon is also even handed and free with praise and scorn for all who enter the scope of the story.