I personally know a guy who’s most likely underwater even though he bought his house with 20% down of his hard earned money. I tried to talk him out of the purchase, too.
Unless he loses his job, he won’t walk away. There’s no point. His monthly carrying costs are MAYBE $500 more than it would cost him to rent a comparable house. Is $500/month worth ruining your credit history?
Don’t assume that everyone who’s underwater will walk.