The faster a bicycle goes, the more stable it is and therefore less likely to fall over. But also when it does fall over, the damage is much more devastating.

You're right that there's a keel keeping real estate from tipping. The reason isn't just psychological (although there is a strong sentimental sense of attachment you have to your house); real estate is also less liquid than almost everything else. Other factors, including proximity to work, children's schools, etc. make it a last resort in terms of liquidating. You're right -- as long as someone can keep up the mortgage payments, they'll do so.

The problem is that the bigger the bubble, the more unlikely someone can maintain those payments if suddenly unemployed or underemployed. The crash of 2008 wasn't so much averted as postponed. Subprime mortgages became 'nonprime mortgages'. The practices are still there. The artificially low interest rates are still there. The bicycle is going a thousand miles an hour and when it really crashes, look out.