I sense Mungo souring on Berkshire as a rock solid long term investment in the American economy.

Not as much as you might think : )
It's probably still the best house on the US block, depending on what sort of building you like. Solid foundations, not sensitive to wind storms, not terribly priced, should hold its value.

The value growth slowdown is predictable, in the sense that the giant size of the firm precludes another "last hurrah" bonus from something like Apple. It's hard to beat the greatest trade of all time, so I assume they won't, and that was the only thing that prevented the inevitable slowdown from showing up sooner. But we have all known for twenty years that a slowdown in the trend rate of growth was coming at some point, so it's hardly a surprise, and is presumably in the price.

In addition, if something happens that makes the US economy 5-10-15% weaker 5-10 years from now than it would otherwise have been, other things being equal, that might mean the same for the value of a share of Berkshire. Yet even if the neighbourhood goes down hill a bit, the best house on the block is still the best house on the block.

For the optimist, Berkshire is slightly "antifragile", to use a term I hate. If the US economy takes a turn for the worse, the equity markets probably will also, so we might see a stretch during which a dollar invested gets a higher number of dollars of future return. I suspect that Berkshire would be one of the few firms prepared, both financially and philosophically, to take advantage of that. So in the "unpleasant outcome" scenarios Berkshire's rate of trend growth might fall somewhat less than that of the broad US economy. "Bear markets are good for business" really should be one of the Ferengi rules of acquisition.

Jim