This idea is not new. During the Romantic era of music (1820-1900) a musician could be involved in at least 5 if not more musical activities in order to make a living. These five options are still around today as well as several others.
A musician could perform - the obvious choice - though it has recently been said, "Many are very good, but few are great, and even fewer have the star-quality that makes for a viable concert-only career." This sounds a bit harsh and perhaps a bit of tough love, but it is the reality for most musicians. Doing something else does not dismiss the option of being a performing musician. It does, however, provide financial stability so that performance can be an expression of one's passion for music rather than a desperate requirement for making ends meet.
A musician could teach often by taking on private students. A fee was usually charged or some sort of exchange took place, perhaps for room and board. Franz Lizst was known for offering free music lessons. This was quite unusual - a virtuoso offering free lessons. I don't know if he just worked with advanced students or beginners, but still it was a wonderful way to give young musicians a great start. The idea of teaching would also emerge in formal school setting, both in K-12 and higher education. During the latter part of the 18th century music teacher training - music education - became more and more important.
Most musicians compose to a certain extent. I often compose for my for my musical ventures - this way music was written especially for the players at their level and for their instruments. All the genres of music that had developed prior to the Romantic eras was composed those usually in a "modernized" expanded manner. New genres would emerge - the symphonic poem, the art song - adding to the possibilities for composition. Chamber music became popular as more and more amateur musicians began performing in the home.
Many musicians would conduct, often their own works. How better to have your own music realized accurately than to conduct it yourself? This offered composers a semblance of control over how their music was introduced to an audience. Conducting did not mean just conducting an orchestra for there were many opera houses and other performance venues needing direction.
A somewhat new vocation for musicians was that of writing music criticism. Critiques of new music, artists' performances, concerts and opera were written to inform audiences about a work or a composer. These critiques commented on the study, discussion, evaluation, and interpretation of music and its performance. Then, as now, critiques could either help or destroy a performer, composer, or conductor's work. The idea of writing about music was not new, but it did become more widespread during the Romantic era. Robert Schumann founded the New Journal of Music in 1834; it continues today and focuses on a different genre of music each issue. Hector Berlioz also would write reviews and criticism of music.
As a result, music students today are trained in each of these areas and several others in their undergraduate studies. This allows each student to find their specialty area while still developing meaningful skills in all areas. And, as they enter the workforce, they are more marketable because of the preparation and foundation they have established.