Moral hazard would be a problem with McCain's proposal, but it's not a necessary symptom of any crisis intervention by the state. People aren't going to make decisions thinking "well, the government will bail me out if there's another once-a-century financial collapse, so I *will* sign up for this credit card!"
Besides, the point I was trying to make earlier was that moral hazard already entered the picture without Uncle Sam's help. Ideally mortgage lenders have a giant incentive to only make loans which are likely to be paid back: the lenders are the ones getting paid back, and in the case of defaults the ones getting stiffed.
Once they're able to convert their loans into opaque financial products, "insure" them, get them rated AAA, and sell them, the lenders are no longer on the hook and feel free to make as many loans as possible, screw the credit evaluation of the home buyer.
And really, we should expect the lendersto keep the terms of a loan sane much more than a potential buyer. The lenders are in the business of home loans. They have a ton of specially trained employees tasked with making these decisions. Between the lender and the buyer the balance of resources, expertise, information, and experience is absolutely, unquestionably, massively assymetrical.
The lender is going to know what each line of the fine print means, both in terms of federal/state law and the likely practical consequences. They'll know what situations are likely to arise in the course of a mortgage and how best to turn those to their own advantage. Every home mortgage negotiation is a professional vs. an amateur, and it's not like there are feasible options for homeownership in our society that don't involve dealing with a large institution and a lot of legalese.
Besides which, there are practical arguments against letting all the people with unaffordable mortgages take a beating. Foreclosures and evictions have a lot of negative externalities, and if you get them en masse you can wind up with some fairly ugly situations.
