I guess the proof will be in the pudding. I recall huge drops in market here in OC 2007 through 2009. There were great buy ops and many foreclosures in our area. Same happened in the mid 1990’s.
2004-2007 was the worst buyer class in history, the job market wasn’t good and folks used their homes as ATMs for every time the price went up. Home equity loans outstanding is lower than 2017, there are 7 million more jobs posted than job seekers right now (vs even in 2007) and this last buying class is the best in US history - read the NY fed quarterly credit report for more. Housing prices nationally did not drop in the 90s more than 1%. SF may struggle as companies move out or CA but nationally there is very little risk to housing due to a massive shortage of housing.