When you are long on bitcoin you almost don't care about volatility. Because you can't predict the future the only good time to buy bitcoin is when you have too much dollars and the only good time when to sell bitcoins is when you have not enough dollars.
Sure, now you have 10 mln dollars in BTC at all tine high, and 2 mln after price drops but in day to day purchases it doesn't matter.
MasterCard could offer joint dollar-bitcoin account that works as follows:
You have some amount of dollars on that account, whenever you pay for something price is deducted from dollar balance.
If dollar balance is insufficient then some portion of bitcoin is sold automatically from your bitcoin balance to give you some multiples of thousands of dollars. You could also have manual trading available if you want to gamble a bit and buy some bitcoins if it dropped to 20% of last ATH or sell some ... I don't know when... when you are drunk and think you can predict the end of the bull run..
Not sure how much bitcoins would someone entrust MasterCard with current bankruptcy law but if it was significant enough amount then all bitcoin millionaires would get to use their fortunes totally hassle free.