The medical definition of conciseness includes things like responding to bright light by constructing the iris. It’s not high on the scale, but it’s very much part of the current definition in active usage. So, in practice it’s useful have level of consciousness and as such the minimum can be extremely low without becoming less meaningful.

More importantly as we understand more about the brain we may eventually understand how everything works. Any definition that would then exclude humans as conscious becomes irrelevant.