Problems in the Definition of Musical Modernism on NG

I just had a class in the literature department today. The course title is Modern Arts and Aesthetics. And so, I looked up on New Grove, and I quote:

Modernism first took shape as a historical phenomenon between 1883 and 1914. Before the death of Wagner, the term ‘modern’ was used interchangeably with ‘new’, ‘recent’ and ‘contemporary’. In its Wagnerian usage it also denoted an embrace of a wide palette of music as a means of conveying narrative and extra-musical content, as opposed to ‘absolute’ music. 

Two composers that came to my mind is Debussy and Berlioz.

Debussy, I think, mainly demonstrates the modernist attitude of negation. He consciously attempts to negate Wagner (although numerous literature have shown Wagner in Debussy's composition), in order to create French music. A more bourgeois type of modernism can be found in Augusta Holmes.

Berlioz's pose a huge problem to the NG description of modernism. Symphonie Fantastique was written in 1830, when Wagner was 17. Symphonie Fantastique is inarguably narrative, as Berlioz explains in his program notes. So, why did Leon Botstein demarcate Wagner as the line for modernism (between modernism and, say, pre-modernism)?

It is worth noting that Beethoven's Pastoral Symphony has descriptive titles, but the logic that works there is that music evokes texts/ imagery, not the other way round. (Vivaldi's Four Seasons is of a considerably different context, and I'll skip the discussion.)

Did Botstein choose Wagner because
- Wagner pushed tonality to its boundary (and soon after him we have post-tonality)?
- Wagner focused specifically on opera (or music drama), and the opera image on stage is a concrete manifestation of the musical rhetoric? (whereas, Berlioz wrote in a variety of genres, and his operas are diverse in style than Wagner's)

The following could be a partial answer

Issues of terminology aside, Modernism, throughout the 20th century, retained its initial intellectual debt to Wagnerian ideas and conceits regarding the link between music and history. The art of music was perceived to need to anticipate and ultimately to reflect the logic of history. In Wagner’s view, the imperative of art was a dynamic originality rooted in the past but transcending it.[...]  Legitimate originality in art was inherently progressive, oppositional and critical.