This is a relatively high concentration to the S&P 500. You've definitely reaped the benefits of higher returns but I do agree with your thought process that some diversity would be good. Although there is a lot of recency bias among investors when it comes the S&P 500 (buying it because it has returned the most in the past) if we were to see a correction among the major holdings in the S&P 500, there would no doubt be some volatility in the index overall.
The most common consensus allocation towards the S&P 500 is around 60%. Many right now may decide to be under weight that because of the overall valuation of the index. Personally, I don't really view 60% as a crazy amount but ultimately it is up to the individual investor. Most all of my US holdings are inside of the S&P 500, and they account for around half of my portfolio.