Slack is crucial for scaling and for organizational health in general.
People with slack have headspace and time to improve things, to think ahead, to sweep out the corners, to prepare for what's coming next. And they will see these needs before management does.
"Running lean" eliminates slack. This only works for a while, and only if you have a flat org of highly self-actualized people who you give autonomy. And you'll still burn people out eventually.