So then purchase and overweight small and mid-caps and international indexes if you think that SPY is top heavy. You don't need to deviate from index investing

So then purchase and overweight small and mid-caps and international indexes if you think that SPY is top heavy. You don't need to deviate from index investing just because a part of the index is outperforming the rest. Choose an allocation, stick to it, rebalance when you hit some threshold.