Looking at various real estate indices over the years, it looks to me like we've recovered through much of the crash, and the FTSE NAREIT price value was higher in 2006 than now, but not in any other year.
Combining that with the fact that many mortgage holders would have bought before 2003, suggests to me that the overwhelming majority of mortgage holders are not underwater, which takes nothing away from the pain of those who are, but numerically, they are a minority. Perhaps half of all mortgages relate to purchases (original purchase date, not most recent finance date) prior to 2003, and I'd wager that well more than half (probably 75+%) of purchases since then have mortgages that are not underwater.
Sure, mortgage holders from lower class families are more likely to have a negative net worth. But that's almost the definition of lower class, not because they're a mortgage holder. Lower class families with last names starting with "S" are also more likely to have negative net worth than average families; has nothing to do with their name...
Totally agree that inflation is the friend of a fixed coupon debtor. I'd be hard pressed to decide to pay off my house even if I had the free cash to do so. It will be much cheaper to pay it off with massively devalued 2024 and 2034 dollars.