I, too, wish that Coinbase would figure out better planning in order that they do NOT run out of coins...
If the price is falling fast, it would be stupid for them to buy lots of BTC beforehand just to be able to sell to people with no delay over the next few days. The loss could easily wipe out all that they make from fees, and more. Better leave the clients waiting.
I believe that is Coinbase's current business decision, in order to NOT be over exposed to downside volatility - however, it may be possible for CB to figure out ways to automatically buy the needed coins on the exchange, at the same time that the customer buys the coin, in circumstances in which they run out of coins. I don't know the exact solution, but I do agree with you that their business decision is to not over expose themselves to downward BTC price volatility.
The problem I'm having with Coinbase is now they want to control volatility on
both sides of the equation: during sharp downturns and sharp upturns. So this means that they will quickly and mysteriously "run out of coins" during the next bubble as well, which will neuter how high it can go by excluding their customers' influence on the market. This is not allowing the free market play out appropriately, it is trying to
control the market, and that is what worries me about Coinbase.