I have one issue with your write-up;
Even if these exchanges run off with tons of BTC, where will THEY exchange it?
If there's a loss of $300 million in a similar form of what happened to Mt. Gox, certain addresses will likely be blacklisted, namely those that happened to get the BTC from the wallets. How they would pull that off, I am not sure. However a large majority of the community is interested in the success of bitcoin, and so I'm sure something will come about.
Only traders/speculators need to keep significant amounts of bitcoins in exchanges, getting IOUs instead. The exposure of regular users is limited to the amount and timeframe of their BTC/fiat deposits/withdrawals.
This is very, very true. Unless you are one of the technologically ignorant, you shouldn't be keeping in excess of $400 in a single wallet, in my opinion. I've had my BTC spread out far and wide, and I still haven't lost anything, amazingly.
By the time Bitcoin is 1 million per coin the exchanges that are centralized will be extremely trustable and with famous faces behind it (Winklevoss with the Gemini one is the start) and not autists like Karpeles.
Also 1 million per coin is not a problem, again, bitcoin being divisible by a shitton etc.
Also a good statement.
I do see what it is that you're trying to say, and I do believe that it's a very real possibility that could cause many issues. I just personally believe that such a scenario is unlikely, and thus there isn't really as need to fret about it.