SP500 is a momentum strategy, and not the best one. So yes, there is a tendency after a long bull run to have a very little number of companies with a very high share of the index.
But the problem is ahead and nobody knows what will happen. Normally stocks tend to have very high correlation in bear markets, a little less in bull markets. But correlation kills the effect of rebalancing. It may take out some risk because of the few big caps.