Exchanges have a sell (cash out limit) of typically a few thousand USD per week depending on verification status, etc., for those trying to trade out fast it is almost impossible. For those who want completely out it will take a really long time to cash out.
I propose a cash out route up to $100,000*.
7% of running average across exchanges + BTC Fee.
This will help miners/investors/early adopters to cash out large amounts of coins fast to use for house/car/hardware purchases.
Why would you even want that? If you really have that much bitcoin to sell then go onto a legit exchange like Bitstamp or kraken and sell your BTC there. Coinbase is more of a vendor type because you can't place buy/sell orders, all you can do is accept their rates at buy/sell terminals.
And what good would letting the miners cash out large amount of coins fast do apart from themselves? As I have already said there are exchanges that you can trade on that accepts infinitely large amount limit orders as long as someone fills it, or you could just place market orders. Absolutely no point in having a higher limit on coinbase when you can go elsewhere with better rates.