Could it be that the ratio of BRK's IV to Book has increased over the last few years? And that's what we'all are missing?

I think it makes sense that this ratio might rise (slowly) over time, but that doesn't mean it has. I remember mungofitch suggesting this back in the 2010(?) era, but it doesn't seem to have changed much since then.

In practice, I'd guess that medium/long term swings in the markets will easily outweigh this effect, making it small enough to be irrelevant. Right now we might be in a swing from growth towards value, which would be expected to raise p/b for BRK.

My thought way back then was that 1.75x would be a reasonable multiple, so anywhere in the 1.5-2.0 area could be expected. More or less by definition, if a company is returning 10-12%/year in modern markets then I think it is undervalued. Another way of looking at it, many other companies would look cheap at less than 2x book, and arguably Berkshire's book value is as solid as most of the market.