A home can be purchased as an investment, as a consuption good, or a combination of both. The reason of the purchase does not take out any risk of ownership.
Ever hear of a bidding war. Sometimes in desireable areas people will actually pay more than the market says for a home because they want to live in that particular house so badly. Thus, sometimes people who buy their home as a consumable good (as you stated) will acutally pay more than the investor. Should the banks guarantee that value?
What is a husband and wife get divorced and need to sell their home to divide assets. The sell is quick and less than they paid for it a year ago. Does the bank need to eat that loss? Same thing with an estate sale if someone passes away? Someone loses their job and needs to move fast and sells for less than market value, should the bank eat that too?
Are you going to limit the number of houses that builder can build or that people can put up for sale on the market at any given time? Is the bank going to be able to tell you when you can sell your home? If it is a bad market, maybe they won't want you putting your house up for sale until it gets better to get you a higher value.
A house is just something that you can't set the price on, or guarantee that it's value wont fluctuate up or down. Your concept just cannot work. There are too many external factors that affect homes values that NOBODY has control over. Building costs, demand, supply, schools, neighborhood, employment, transportation, interest rates, population, amenities/entertainment, how people in the area are maintaining their homes, etc. There is no way that a lender or anyone could guarantee anthing like this. You really need to step back and look at what you are saying. The real estate industry is an open market that cannot be set by a handful of individuals. It is set by millions of buyers and sellers and other factors.