Rimsky-Korsakoff didn't stop composing but he had an interesting crisis or composer's block that perhaps sheds light on the issue. He was one of the gang of 5 Russian composers (I'm writing this all from memory without looking at sources). He had several successes but decided he needed to firm up his music theory because he thought it would make him a better composer. A Russian conservatory made him a professor before he was ready, and he was taking musical theory classes at the same time he was teaching music! When he had mastered the analytical part of music, namely theory, he found that it paradoxically had the opposite effect on his ability to compose. He lost the intuitive muse, or inner voice or inspiration, that he once had. The musical ideas didn't from flow to him through analysis, but through intuition. So he had a lull in his compositional output; fortunately he was able to pull himself out of the slump and begin composing again with an effective balance between the analytical and the intuitive, but a lesser individual with this artistic crisis may have quit. In fact, Rimsky-Korsakoff helped other composers such as Mussorgsky with his broadened and deep knowledge of music theory. What's also interesting about Rimsky Korsakov is that he had a long-time friend and biographer who wrote a very detailed and anecdotal account of his life, with his friend purposively recording conversations during his visits. Another prominent example of this from non-modern times is Boswell's biography of Samuel Johnson. Rimsky Korsakov also became a master of instrumentation due to his musical job with the Russian government that caused him to be very familiar with all the instruments. We see that with Capriccio Espagnol. But I think we would need a close friend who had been observing Igot Markevitch to see what artistic block, etc may have occured at age 29. Maybe he ran out of intuitive inspiration and musical ideas and didn't have the theory to sustain himself; maybe he developed new interests; maybe he didn't want the pressure of what others might expect from him so that he couldn't be genuine to his muse. There are one work wonders or artists in the twentieth century. One is Bobby Pickett who composed the Halloween classic, Monster Mash Song. The other is the writer, Harper Lee, author of To Kill a Mockingbird. As Herman Melville said in one of his poems, wrestling with the angel art is a very difficult proposition, especially to wrestle all your life.