My cynical 2 cents
The Housing Bubble like many successes and failures has many fathers.
More regulatory oversight (and a lot of regulatory forward thinking about CDOs, derivatives trading et al), maintaining Glass-Steagall preventing the casino atmosphere of banking, and RICO prosecutions of corporate criminals might have resulted in a lot softer landing, and yeah, making it clear that 20% down was the minimum to get a mortgage might have cooled down the frenzied speculation.
It became your classic dumb money-chasing- dumb-money speculative lemming rush that hasn't changed since the Dutch tulip craze. Institutional investors jumped in thinking the Feds would backstop it via Fannie Mae and Freddie Mac who were greedy as everyone else believing gravity had been suspended. Having them say, "Sorry folks, market's too yeasty, the property's actual value's nowhere near what we're underwriting." would've sent shock waves through the system that would've made everyone reevaluate risk, but no such luck.
Same thing with the Fed raising interest rates. The lenders didn't want honest risk assessments. Nobody in local, federal or state government wanted to kill the golden goose that was showering them with revenue without raising taxes and so it went.
However, the elephant in the room is that for 98% of the US working population, incomes have been flat or declining in real buying power since 1980. That's due to a variety of factors, union-busting and general job insecurity that have made the US income distribution ever-more-skewed to the top 2% who have the spare cash to invest. Everyone else has to scrape by as health care, tertiary education, and retirement costs eat up more of our income every year. The only way folks maintain the expected lifestyle is working themselves to death and accessing and using credit to the limit.
Most of the folks flipping houses were rich to begin with, they just found a low-risk speculative enterprise that made them richer quicker and easier than gambling on the stock market. You had a few slick operators that played the system like a fiddle with no money down and became rich, but those were the exceptions proving the rule.
So what would've prevented this is average Americans accepting that houses cost a lot and aren't investment vehicles accumulating value but where they build a life because it suits their needs and what they can afford for ten years or more.
Average Americans need a situation that makes that strategy worthwhile.
They need stable, good incomes and work situations that don't force them to move every few years, though people move for all kinds of reasons.
Opportunities sometimes are thin on the ground where you are and it makes sense to move to a place where you've got a better shot. Balancing the goodies you get for going elsewhere/ doing something else vs the goodies where you are isn't a magic formula that applies to everyone.
Still, the American economy is still in love with the idea of entrepreneurship where pluck, luck, and hard work can make you rich. They can, but the odds suck. We need a strategy for other 98% that doesn't continue our current race to the bottom where the only individual hope is a lotto win. Socialism has its place when a public interest exists to provide a set of services to all that improve the public good, but that's a very slippery target. Who establishes that consensus of priorities and willingness to make those happen? Unfettered capitalism creates a Social Darwinist hellhole for all but the privileged class and their lackeys. The answer may be somewhere outside both concepts.