Nothing you said negates ML as a possible solution to better tailor content for people. Other platforms like Twitter, Facebook, Instagram, etc. already use algorithms to try to better tailor content for users to increase engagement, absolutely no reason Reddit couldn't offer something similar (with the option to use the classic mode obviously). Also to be clear I'm talking more about comment quality than post quality (there are no ads in comments).

Of course using length of comments isn't perfect, and that should never be the sole input in any general-purpose algorithm, just one variable among many. I still believe it could be used to help improve quality. It would likely be more about punishing extremely short comments (eg. one or two sentences) which are rarely insightful, rather than simply scoring comments based on length.