Nvidia has gained $1.9 trillion in market cap since the start of 2023 -- but the sledding is about to get tougher.Over the last three decades, Wall Street has entertained no shortage of next-big-thing investment trends. Since the advent of the internet changed the long-term growth trajectory for corporate America, investors have been waiting for the next innovation that would alter the business landscape. The rise of artificial intelligence (AI) might just be the unicorn the investment world has been waitin ...