I have been listening to a bit more mid-20th century American music than usual of late, and have been following (and participating in) discussions about some of the various musical values that composers embraced during the last decade. I have a couple of very close friends who acted as pioneers in the New York new music scene during the 1960s and 1970s. They commissioned and performed pieces of new music (sometimes without the exchange of money, and sometimes using funds supplied by third-party backers) for audiences of people who were interested in expanding their horizons.
These audiences weren't ready to do any kind of evaluating, but they were living through some of the huge extra-musical cultural shifts that were happening during the 1960s, and they were curious and open to new things. Some of the most revered and admired composers of the time were indeed charlatans, and some of the competent ones were a whole lot smarter (intellectually speaking) than most mere musical mortals. Some were interested in developing and using technology to generate music electronically, and some were interested, because of what they learned from working with electronic music, in finding new ways that acoustic instruments could make sound, which often provided great challenges for performing musicians.
Some performing musicians, who were parts of established ensembles, performed this music because they had to. My father used to refer to the new music he often performed with the Boston Symphony Chamber Players as "Mickey Mouse Music." But then there was new music that was not "Mickey Mouse Music." The big question (and challenge) for musicians was really to find the music in the music, and finding the music in music that is not organized tonally, and is not organized in 12-tone rows or is a mixture of tonal material and tone rows, takes a great deal of creativity.
In other words, it takes a great deal of emotional and intellectual voltage to glue an array of seemingly-random pitches together horizontally (and sometimes vertically) in a way that makes some kind of sense, and a successful performance is truly a mixture of the performer(s) and the composer in at least equal proportion. Depending on the piece, it can even be a 70/30 (or greater) split between the performer and the composer, favoring the performer.
Some composers tried to remedy this situation by writing very specifically-notated and complex music that would keep them in control, but the performing musicians who commissioned their music would still figure out ways of inserting their particular creativity into a performance. This fight (or dance) between composer and performing musician can be fascinating and exciting, and I believe that part of what we respond to as listeners is the dynamics of that relationship. Unlike performances of music in the usual canon of the time, these 20th-century composers were not only living, they were often at the performances.
New music has certainly changed in the past 50 years or so. Now some composers tend to tailor their compositions to the "playing" of a particular musician (we have seen this in a few recently-commissioned high-profile violin concertos), because composers have finally figured out that the best way to gain commercial success is to have a connection with a hot soloist. Hot soloists have also figured out that they can increase their public profiles by commissioning music from hot composers (who will tailor their work to fit the particular qualities of the soloist). These relationships are often born in high-level conservatories, which makes the high price of admission something to take into serious consideration for both composers and for people interested in playing new music.
I could easily be proven wrong, but I believe that this kind of "hot" soloist playing new music during the 20th century was still more about the music than the soloist.
I have certainly tried, but I have never been able to write much in the way of meaningful serial music. But the lessons of the 21st century tell me that new and meaningful music does not need to be limited to the abstract and obtuse, just like new and meaningful visual art does not need to be limited to the abstract and the shocking. I'm happy to relinquish control of what I write to whoever performs it, and tend to give very little in the way of performance directions (though I use pitches, rhythms, articulations, and dynamics, and make sure that the phrases are obvious, logical, and self-directing). Some new music people (who cut their musical baby teeth listening to the huge array of what was new music in the 1960s) might consider what I write to be uninteresting, because it doesn't compel them "connect the dots" in order for the music to make sense, but there's really nothing that I can do about that. I guess they have to figure out how to apply their creativity to the music I write the "old fashioned" way.