Can one person succeed at a serious and dedicated effort to withdraw twelve billion dollars USD from Bitcoin at once?
If it succeeds, cool. If it fails, it'll be spectacular. Let's do it. (To me, success is "they get a few billion dollars USD out and bitcoin doesn't crash".)
This seems pretty easy to test. Just go through all the exchanges and sum up their orderbooks. No need to actually test with 259684 BTC and lose a bunch of money through slippage.
@Bitfinexed (https://twitter.com/Bitfinexed) would argue that order books based on USDT (Tether) are not, and either the rug would be pulled or you will quickly find that you're holding ~2% of filled value (actual backed Tether)
If you want to sell this much you really do not do it on screen (and definitely not at once). You get a big OTC desk to sell in tranches.
This is often not a problem for fiat currencies, they have an entire financial sector dedicated to buying and selling a billion dollars at a time. They are slightly demanding from a regulatory perspective, relative to Bitcoin, but they also do usually try not to crash the currency and they do sometimes get it done.
So I consider that ability to cash out a billion dollars at once a direct spot-check measure of Bitcoin's financial maturity. If this is actually possible to transact at all, then it proves something competitive and positive about Bitcoin, even if it's as much about currency exchange and financial instrumentation businesses as it is the blockchain. I think it is unavoidable that we'll evaluate this through a real-world event, what with one of those exchanges having IPO'd recently. I'd just like it to be an intentional test rather than a surprise, from someone who isn't trying to destroy the world, but does require detailed financial records and a written declaration of insolvency in order to forgive payment.
On the other hand I have no doubt you could realize nearly all of 1B in BTC using proper trading strategies/algorithms over days or weeks, so I don’t see what would be learned from the test except “how deep is BTC liquidity currently” which is kind of what the market is trying to determine all the time anyway.