In a perfect world, each child would find suitable challenge in the classroom everyday. They would be working "at the edge of their ability" across multiple subjects, enjoying hard-won successes and learning to re-double their effort when they've failed to master a new skill. The inherent reward of hard work and the feeling of progress keep students engaged. In the real world, classrooms, by definition, have mixed-abilities, and if the students are in a traditional classroom, the teacher is pitching the lessons to the students in the middle. For a bright kid, the pacing or level of material can be mind-numbing. For a really bright kid, the pacing, the level of material, and the "spiraling" repetition in the elementary school years can be absolutely torturous. It is not at all unusual to have a bright kid "call it in" during the first 7-8 years of school, never having to exert themselves much or spend much time on "homework". And then they get clobbered by the demands of high school...they've never learned the skills and mindset needed to get through challenging classes.
So. back to Kindergarten. The problem when you're making these decisions to send or not to send is you don't know what kind of kid you have. You might have an idea, but maybe not. Typically, you can't get perspective on your own child and kids are still changing a lot at 4 and 5 years old. As a parent you typically don't have any yardsticks to help you make these decisions, unless you've had some educational testing done, either identifying IQ/achievement strengths and weaknesses or identifying specific issues your child might have (e.g., dyslexia, dysgraphia, ADD, ADHD, etc).