I understand your reasoning, but it is based on incorrect assumptions. The first assumption you make is that we actually know how pre-18th century music sounde

I understand your reasoning, but it is based on incorrect assumptions. The first assumption you make is that we actually know how pre-18th century music sounded like. We have guesses, some better than others, but we know that we are likely wrong. The second assumption is based on the idea that music somehow improves in complexity and quality. We have many scores that have survived the test of time, but these provide a huge selection bias. Only scores that were popular, widely copied and preserved due to timelessness are currently available. Musical ideas that could not be expressed using the scoring techniques didn't survive. Nor did musical ideas that didn't mesh with following eras. Therefor, what we see is a pattern of scores that are popular (of low complexity), easy to write (of low complexity) and similar to following eras (of lower or equal complexity). See the selection bias? Lastly, and more concretely: 18th century musicians didn't suddenly start caring about bending time. They started caring about sharing musical ideas outside of their direct sphere of influence (printing press), so they understood the need to add extra markings on sheet music, such as rubato and accelerando. And saying you try to understand jazz is akin to saying you try to understand philosophy. It cannot be done, too much has been written and one cannot understand all standpoints simultaneously. Try to understand a specific era, like Bebop, or a specific musician, like Chick Corea.