The calculation now for our couple with $100k household net income isn't all that bad. The monthly "rent" for the house is between $2k and $3k of which $1.2k is down payment ($24k-36k year). Double the interest rates from 2% to 4% and it's $3k to $4k/month. I think spending over 50% your net income on living costs is proabably a limit to look out for - and that will certainly happen to these people should interest rates go north of 5% or there about. We also need to remember that in Sweden people generally don't save up for a rainy day. I'm not saving for retirement, kids edication, risk of being ill etc. So in a nanny state the equation is slightly different than in say the US.
> Which sane person who is good with money does that?
It doesn't matter so long as it is at least one person in the auction for each house does it. If there are no homes to rent and nothing is built, there will be competition for the existing homes on the market.
> not unrealistically, an underwater house during your lifetime
This is definitely a risk. And banks are now lending less and less. With a 25% down payment the bank will know the hose can take a 25% correction and still be over water. But the scary bit is of course that if there is a 20% correction, no one can move, and no one will consume anything for a very long time, which will lead to pretty significant economic fallout.