My theory is that when European classical music was being written, there were no loudspeakers, amplifiers, effects or digital instruments. To fill whole auditab

My theory is that when European classical music was being written, there were no loudspeakers, amplifiers, effects or digital instruments. To fill whole auditable frequency spectrum multiple instruments were needed to play multiple pitches at once. Reason for this is that most instruments could only fill part of the spectrum. Organ being one notable exception. Still, even on organ, to fill the whole spectrum, multiple notes need to be played at the same time. So, harmony was very important to produce a full sound. Today, a single synth can fill the whole frequency spectrum by playing just one note. So, to have a full sound, harmonies aren't necessary anymore.