I figure it this way.
If you buy a house and the value goes down you walk away. The bank gets the loss.
If you buy a house and the value goes up then you sell the bank should then get any profit made from the sale. If it goes one way to be fair the sharing or value change should go both ways.
In this reality. Pay for the mortgage on the house you did everything you could to obtain. You asked for it. Now live with what you got after requesting the ability to obtain it.
The bank should NEVER be responsible for any gain or loss in equity you achieve. That would be unfair as they only gave the loan you asked. If we socialize losses, it would not be to long before we socialize profits also.