I wonder if proponents of OO truly understand what such a system would look like, especially if you tried really hard to adhere to tenets of good design, like reducing coupling, increasing cohesion, etc.

It would be a LOT of classes that all do little things, grouped into namespaces. A LOT. No, more than that.

If you wonder how JavaLand ends up with stuff like https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/aop/framework/AbstractSingletonProxyFactoryBean.html that is how. Complex system require a lot of stuff to manage that complexity.