That sounds great in theory, but you can't implement it in practice. It's often easy in hindsight to identify where are those suboptimal 5%-away-from-perfect-timing points where you should have done a rebalancing, but we have no idea how close we are to the next peak/trough.
You also have to get back in at the right time. A lot of people called that the market would drop in March 2020. But how many realized that by August, long before vaccines were in sight, that the market wouldn't go below the pre-covid peak again? Somebody who sells in March and then buys in October loses a lot.
