Does the collapse of Bitcoin present any kind of new-fangled risk for traditional investors in actual securities? Does it depend on how high it goes before the fall?

Interesting question.

To simplify a bit, it's a zero sum market. If the price of Bitcoin collapses and even disappears, it is in effect just a big transfer of wealth from one group to another. A lot of money would have been moved, net, from small punters late to the game and dumb institutions and Microstrategy shareholders to some hucksters and thieves and early adherents who lightened up (and a few smart but endearingly fearless folks who are short), but the total amount of wealth in the world would be entirely unchanged. The amount of money moved (net) from one group to another is vastly smaller than the nominal "market cap" of bitcoin.

The wider damage would be limited to the secondary fallout from the losers. Lower purchasing power among people who just lost their savings, defaulted-on bondholders of Microstrategy, and so forth. I think this would be unpleasant in a few places but not at all Lehman-like. It could have secondary "reflexivity" effects on sentiment. It would be a hit to the wealth effect (a certain population would feel poorer and react accordingly), though not to aggregate actual wealth.

In terms of markets, one would not want to be long non-bitcoin things that aggressive bitcoin holders tend to own a whole lot of. Some will have bought on margin and will sell whatever they can, causing the prices of those things to come under pressure.

Jim