I find it telling that RT uses the word “monumental” here. It implies a fixity, a concreteness; monuments last beyond the puny scale of their human creators. In the late 18th century, symphonies were anything but this – they were music for social use. A monument, on the other hand, isn’t terribly functional; rather, it’s meant to be gazed at and admired for its own sake and for what it represents. So what does the monumental 19th-century symphony represent? Emancipation from the social? Autonomy? A paean to extreme individuality? Universality?
Perhaps a monumental 19th-century symphony is one so well written that an performance of the piece is the piece itself and not just a performance of something that has the same notes, rhythms, orchestration, and harmonies of the piece. I like the word “monument.” Brahms’ Second Symphony (and all his other symphonies, for that matter) have not changed since my childhood. A good performance of it is just as satisfying now as it was twenty years ago, thirty years ago, forty years ago, or one hundred years ago. The size of the orchestra doesn’t change, the basic approach to playing doesn’t change, the conventions of performance (not clapping between movements, for example) haven’t changed (yet).
The 19th-century monumental symphony represents, above all things, compositional competence, understanding and application of form, and the understanding of how to use the orchestra as an instrument.