Have a look at this:
It's probably not the greatest explainer because it's not really focused on rebalancing and it might have been useful if they added in what things might have looked like without the rebalancing, but I guess that wasn't the topic of the video, but it does show a real life example.
Check the growth of wealth chart they put up, MSCI World was negative through 2000-12 I think, while the ASX did probably +7% pa. Big divergence, but as you say, the winning ASX was being sold and those proceeds were buying the losing MSCI World. So it would have kept the allocation the same, as we know, the MSCI World started doing double digit returns in the 2010's and became the winner. Because the allocation was rebalanced, the MSCI was still 60% of the portfolio when it started to outperform, instead of who knows? 30%. So I imagine it helped a little with performance in the last decade, you could also probably argue it was a drag for all those years, but at the same time you'd wonder how many people wrote off global stocks and didn't touch them after that and missed that boom?
The point is none of us are Warren Buffett, and we don't know how long anything will win or lose, but rebalancing keeps the portfolio inline with our original intent when we started.