Musical eras seldom disappear at once, instead, features are replaced over time, until the old is simply felt as "old fashioned". The classical style did not "die" so much as transform under the weight of changes.
One crucial change was the shift towards harmonies which center around "flatward" or subdominant keys. In the classical style, major was far more common than minor, chromaticism controlled through the use of "sharpward" introduction of keys, and minor sections were for contrast. Beginning with Mozart and Clementi - there began a creeping colonization of the subdominant region. With Schubert, it became a full fledged land rush: with subdominant moves being substituted in places which previous composers would have used strictly dominant regions. (For a fuller discussion of these terms see Tonality). This created a darker color to music, strengthened the minor mode and made structure harder to maintain. Beethoven would contribute to this, by his increasing use of the fourth as a consonance, and modal ambiguity - for example the opening of the D Minor Symphony.
Among this generation of "classical romantics" Franz Schubert, Carl Maria von Weber, John Field are among the most prominent, along with the young works of Felix Mendelssohn. Their sense of form was strongly influenced by the classical style, and the were not yet "learned", imitating rules which were codified by others, but directly responding to works of music which they knew by Beethoven, Mozart, Clementi and others. The instrumental forces at their disposal were also quite "classical" in number and scope, leading them to compose parts which were similar in the way they were played.
However, the forces which would end the hold of the classical style grow in the works of each of these composers. The most commonly cited one is, of course, harmony. However, also important is the increasing focus on having a continuous rhythmically uniform accompanying figuration. Beethoven's Moonlight sonata would be the model for hundreds of later pieces - where the shifting movement of a rhythmic figure provides much of the drama and interest of the work, while a melody drifts above it. As years wore on, greater knowledge of works, greater instrumental expertise, increasing range of instruments, the growth of concert societies, the spread of the piano - which created a huge audience for sophisticated music - all contributed to the shift to the "Romantic" style.
Drawing the line exactly is impossible: there are sections of Mozart's works which, taken alone, are indistinguishable in harmony and orchestration from music written 80 years later, and composers continue to write in classically normative styles all the way into the 20th century. Even before Beethoven's death, composers such as Louis Spohr were self-described romantics and incorporated more and more chromaticism in their works. However, generally the fall of Vienna as the most important musical center for orchestral composition is felt to be the moment where the classical style, with its continuous organic development of one composer learning in close proximity to others, finally ended. Franz Liszt and Mendelssohn, as well as Fredric Chopin, visited Vienna when young, but they then moved on to other vistas. Composers such as Czerny, while deeply influenced by Beethoven, also searched for new ideas and new forms to contain the larger world of musical expression and performance in which they lived.







