the answer as to why rebalancing can hurt you is over typical accumulation time frames which span decades not just years , equities have always been the best performers . so with markets typically up 2/3's of the time and down 1/3 you are taking money out of what typically sees the most gains over the long haul and putting it in things that see less gains over long periods of time and may just be the best performer over much shorter years . .
without rebalancing you will be on a constant rising glide path that may become much to volatile or long term sensitive and be a poor choice for counting on short term money from it comfortably .
as michael kitce's found , in order to do better you really have to be a good market timer . it is more for keeping your plan in a "mental allocation " at a point you can deal with rather than a good financial idea based on performance .
Executive Summary
The conventional view of portfolio rebalancing is that it is a strategy to enhance long-term returns by periodically selling the investments that are up (and overweighted) to buy those that are down (and underweighted), in the process of realigning the portfolio to its original target allocation.
Yet the reality is that because most investments go up far more often than they go down, systematic rebalancing is actually more likely to just consistently liquidate the best-performing investments to buy ones with lower returns instead – especially when rebalancing across investments that have very significant return differences in the first place (e.g., rebalancing from stocks into bonds).
As a result, rebalancing may be helpful as a risk management strategy – otherwise higher-returning stocks would compound to the point that they are significantly overweighted relative to lower-returning bonds – but it’s only when rebalancing amongst investments with similar returns in the first place that rebalancing provides a return-enhancement potential.
Ultimately, the fact that rebalancing may actually reduce long-term returns isn’t a reason to avoid it (even if returns are lower, risk-adjusted returns may be improved if the risk is reduced by even more), and sometimes returns really can be enhanced (when rebalancing across similar-return investments, such as amongst sub-categories of equities). Nonetheless, it’s crucial to recognize the role that rebalancing really does – and does not – play in a long-term portfolio!
https://www.kitces.com/blog/how-rebalan ... nt-anyway/