We’re missing 3 ingredients from 2008: adjustable-rate mortgages, high risk mortgages (0 down, interest-only, no doc, etc.), and unemployment.
Most mortgages written in the last 5 years - that is, the ones which might be underwater - are fixed rate and have meaningful down payments.
Even if unemployment rises[1], as long as homeowners still have the same monthly payments, and have a meaningful down payment, they’d stay in their homes as long as possible.
In cities where appreciation has been strong, I think the likely outcome is a small (5-15%) decline over a few years, then flat appreciation (ie, a decline in real dollars) for a few more years, and relatively few sales because most folks would stay rather than sell while underwater. This may decrease geographic mobility. Basically, when rates rise closer to historical norms, 5 years of appreciation is already priced in and anyone who doesn’t need to move doesn’t move.
As with most assets, don’t put yourself in a position where you absolutely need to sell.
[1] If mortgage rates go up significantly, employment is still probably relatively low or the Federal Reserve would loosen its monetary policy and that would affect the mortgage market. I’m ignoring the wildcard case where the Fed’s monetary policy moves no longer have any impact.