Yes, AUM is a different matter. Rebalancing as a thing came about to get some profit from accounts just sitting there. It was probably great money right up until fees finally stopped. Early trading fees were much much higher long ago. It was good money to scare people from stocks to bonds.
Today, rebalancing is embraced as risk reduction, such as in your case. While I don't personally subscribe to it, I can understand how asset shifts over time give people peace of mind. Yes, stocks are the riskiest of investments (and best potential long haul reward). However, put into context, there is a spectrum. P&G is hardly a "lose everything you have in it" type of investment risk. Sure, if they hit the big skids, shareholders are last. I can live with that risk easier than I could stomach going from PG shares to their bonds. I personally do not need the 'almost certainly a sure thing' bond return and will take the far-from-guarantee path.
Tax strategies are also a consideration, and rebalancing can definitely play a role there. Anyone needing to take money out most certainly has an eye towards where to take it from and what that means to the overall. I would say that I would be in favor of shifting things around when something forces a change (like withdrawal or sudden life change) but I would not ever rebalance on some schedule for sake of rebalance.