I'm sure it will happen, but the price will have to be stable first. And that will happen when bitcoin has billions of users, not 2 million with wild volatility.
Otherwise the bank would accept the collateral, the person would fall through, and the bank will find out that the bitcoins are only worth half of what they thought.
Yes, of course, any asset valuation is discounted because of market volatility or liquidity concerns. But as you hinted, I expect the 'Bitcoin as collateral' story to be a good one in the near future for many banks and real estate firms.