Rrr12345, I'm not sure I agree the S&P's growth rate dropped suddenly post-1999. There was a serious market downturn from 2000-2008, but that's just a normal part of market action over the long term. If you calculate the growth rate starting at the peak (2000-2023), you'll get a growth rate substantially below the long-term average. Doing the same calculation starting at the market bottom (2008-2023) will give you a number substantially higher than the long-term average. There's no clear evidence the long-term growth trendline has changed, at least in my eyes.

That said, I agree long-range forecasts have their issues and you need to be careful when using them. If we ended up having a 1929-style depression (or worse), the long-term forecast is going to be garbage. This is unfortunately something difficult/impossible to anticipate, so that is a risk factor for not only Berkshire, but pretty much everything out there.

Another risk factor is Berkshire's underlying business model changes substantially. This is something that has happened in the past (e.g. amount of money being invested) and could easily happen in the future. Fortunately, this is something that often happens slowly and you can watch it unfold and anticipate its possible effects well in advance. Having time to adjust makes it much less worrisome. I've thought a lot about what will happen when Warren and Charlie leave Berkshire. I have no idea what the stock price action will look like, but I think it will have very little effect on the stock price five years out. Berkshire is a huge earnings battleship that will continue moving reliably forward long into the future. Any attempts to change course will take a long time before they have a substantial effect. There will be ample time to notice and you can make adjustments long before they have any dramatic effect on the stock price.