"The procedures of Haydn and Mozart must be understood in a larger context, that of the creation of a popular style which abandons none of the pretensions of high art ... Their achievement is perhaps unique in Western music ... Only for one brief historical period in the operas of Mozart, the late symphonies of Haydn and some of the Schubert songs, has the utmost sophistication and complexity of musical technique existed alongside--or better, fused with--the virtues of the street song." [p. 332]He goes on to discount the objections that we find popular and folk tunes in Renaissance music, Mahler, Bartók and other places by making the valid point that they are intrusions into the fabric and structure of the music, not fused with it. In the case of Bartók, what he took from them were non-diatonic modes and some rhythmic ideas which he used to create a modernist style NOT fused with a popular style.
What I take from this are a couple of interesting points. One is that, contrary to what we often seem to think, high art music is usually remote from popular music in music history. Those periods when it seems to come into the more popular culture are atypical. The Disney movie Fantasia was a success because of the relatively brief existence of a "middle-brow" culture in the US.
Most so-called "classical" music has been more or less esoteric from the beginning. In the darker part of the middle ages, before 1000 AD, chant was probably the most sophisticated musical form and to the ordinary people, it was undoubtedly "esoteric". They certainly had their own popular styles of dance music which were not written down. For most historical periods, these two things, popular music and esoteric music, co-existed without mixing. There were glimpses of fusions of popular and esoteric styles that Rosen does not mention, such as some 16th century madrigals. Doesn't this have a popular appeal to it?
We hear there the fusion of infectious rhythms and a pleasurable text with sophisticated harmony and counterpoint. This kind of union of the popular and the esoteric is, as Rosen says, uncommon. The late-18th century style of Haydn, Mozart and Beethoven, while not quite unique, is certainly rare and probably the most complete and extensive repertoire in which this is the case. It forms the core of what we now call the "classical" repertoire or the "canon". Many of the awkward difficulties that have come along later, such as trying to define "classical" or fit what modern composers are doing into the model, come from the nature of this founding repertoire: it was a fusion of the popular and the esoteric.
Mozart could write a piano concerto full of delightful rhythms, charming melodies and piquant harmonies that everyone could enjoy and combine that with passages displaying feats of learnéd counterpoint for the more sophisticated. And he could do this with ease because the classical style could accommodate this. Few other styles could.
I mentioned the late 16th century madrigal as an example, but I think there are a couple of twentieth century ones as well: some genres of jazz and some genres of rock could be seen as successfully fusing the popular and esoteric. Here are a couple of examples of those:
Now you could certainly argue that neither of these are entirely successful, the Ellington because none of his longer more complex compositions was ever accorded the acclaim of his 3 minute masterpieces, and the Beatles song because it is not "sophisticated" in traditional ways. The mastery shows itself rather in the details of the recording more than in things like counterpoint. But I think you would have to admit that these are at least attempts at fusing the popular and the esoteric?
I can think of one more example where the popular and esoteric seem to meet: the tango music of Astor Piazzolla:
The question is, why is this so rarely attempted? Most musicians, both classical and popular, tend to stick to their own side of the street and not wander too far outside the perceived limits of their genre. When they do, it is often seen as a circus trick. This might often be because fusing the popular and the esoteric is much harder than it sounds. Mozart makes it sound easy, but these guys show us how hard it is:
The problem often is that the most appealing elements of a popular style, such as the rhythmic swing or the funky bluesy lead guitar, do not fit well with typical orchestral textures and timbres. Oil and water don't mix well! The composer has to dig much deeper to find the base on which both the popular and esoteric elements can meet. As Rosen is saying, it is doubtful than anyone has done it better than Haydn, Mozart and Beethoven.