Arnold Schoenberg considered the idea of twelve equidistant pitches to be a temporary compromise.

What did Schoenberg say about microtonality?

Schoenberg's opinion on microtonality was succinctly summarized by Roy Carter in his English translation of Schoenberg's Harmonielehre (1921):

Finer subdivisions of the octave Schoenberg did not accept as, at that time, [1921] technologically feasible or culturally necessary.

This conclusion is drawn from two passages. The following is a continuation of the excerpt quoted above:

"Then our scale will be transformed into a higher order, as the church modes were transformed into major and minor modes. Whether there will then be quarter tones, eighth, third, or (as Busoni thinks) sixth tones, or whether we will move directly to a 53-tone scale that Dr. Robert Neumann has calculated, we cannot foretell. Perhaps this new division of the octave will even be untempered and will not have much left over in common with our scale. However that may be, attempts to compose in quarter or third tones, as are being undertaken here and there, seem senseless, as long as there are too few instruments available that can play them. Probably, whenever the ear and imagination have matured enough for such music, the scale and the instruments will all at once be available. It is certain that this movement is now afoot, certain that it will lead to something. It may be that here again many digressions and errors will have to be overcome; perhaps these, too, will lead to exaggerations or to the delusion that now the ultimate, the immutable has been found. Perhaps here, once again, laws and scales will be erected and accorded an aesthetic timelessness. To the man of vision, even that will not be the end. He recognizes that any material can be suitable for art — if it is well enough defined that the imagination has no unexplored territory left in which to roam, in which to establish mystical connection with the universe. And since we can still hope that the world will long continue to be a riddle to our intelligence (Verstand), we can say in spite of all Beckmessers that the end of art is not yet at hand." [pp.25 - 26, itallics added]

The other relevant passage appears in the Appendix:

"One may assume that finer subdivisions of the octave into scale degrees (Mehrstufigkeit) indicates a higher level of development. Then, the greater number of available scale degrees yields so many more melodic possibilities that, even with its greater age, music embodying such has probably not yet had time to advance much beyond the elaboration of monophonic combinations. Hence, there all polyphony is probably at best in its early, tentative stage, comparable to the initial stage of polyphony in our music several centuries ago. In the meantime our music has rather exhaustively exploited the possible relations of seven tones, not just in one voice, but in several voices, and with the concurrent refinement of motivic logic besides. And now our music is about to attempt the same with twelve tones. Let it be freely admitted that the same [ideas, feelings?] can be expressed with more primitive as well as with more advanced combinations. But that must then also hold for us, and our music could then advance only by widening the circle of our thought, of our ideas. It should of course not be overlooked that a progression of tones is in itself, to a certain extent, a musical idea and that the number of such ideas increases with the number of available tones. Nevertheless, twelve notes squared by the second dimension, polyphony, presumably yield just as many combinations as twenty-four tones that are combined monophonically, in only one dimension. There are enough possibilities, at least, to postpone for some time any necessity for further subdivision of the octave." [p.424, itallics added]

Was Schoenberg correct?

In hindsight, we can see that Schoenberg was correct to predict the postponement of a shift to a microtonal paradigm in Western music. He correctly considered that the possibilities of twelve pitches should first be thoroughly examined and exhausted before microtonality would become the next logical step. Schoenberg had a clear sense of his own place in history. During the 20th century, the idea of twelve equidistant pitches was taken to its logical conclusion, almost as a direct result of Schoenberg's influence. He saw the potential of expanded pitch resources, but he also knew that, however useful something is, it will only flourish if its time has truly come:

"To predict and influence the future, even more than to be modern, a sense of timing is important. ... It is not merely tone that makes music, but timing (das Zeitmass) as well; and it is typical of dilettantes of all fields and tendencies that they are devoid of all feeling for at least one or the other — tone or timing." [p. 425]

In Schoenberg's day, the time had not yet come for a paradigm shift to microtonality. Today, Schoenberg's dictum has been fulfilled; we have in fact experienced the exhaustion of the possibilities inherent in twelve pitches. We have also experienced more than a century of practical experimentation with expanded pitch resources. If the cultural acceptance of greater numbers of pitches depends upon historical necessity, then microtonality is an idea whose time has clearly arrived.