They already have two "heir apparents" - Greg Abel and Ajit Jain. Both are taking over some of the investment decisions (it's thought one or both are responsible for Berkshire's position in Apple, for instance.) Buffet has made it pretty clear one of the two will move into his role, with the other perhaps taking over Charlie's position.
I've owned Berkshire for more than 20 years, continued to add over the years and have never sold. You are correct -- it has essentially tracked the S&P 500 for the past ten years, and in some cases slightly underperformed. It's now so big and so diversified I think it's days of crushing the index are behind it. Plus, value investing in general has trailed the S&P 500, which is more and more driven by FAANG stocks and other tech names.
I do think the makeup of Berkshire's companies and holdings mean it will not fall as far as the S&P 500 will when this bull market inevitably comes to an end. Plus, Berkshire is sitting on $112 billion in cash, and when the bear does roar, history has shown Berkshire is exceptionally good at deploying that cash to buy up bargain stocks or entire companies. Plus, I love the way they manage to structure sweetheart deals like B of A a decade ago or the recent Occidental deal.
Will this record continue without Warren and Charlie? I would like to think their DNA is thinking is so ingrained in the genetic makeup of Berkshire that it will, but I guess we'll find out. Obviously you know how I'm betting.