Really, I think I'm making three incorrect assumptions:
1. People that buy a house will stay in it for 10+ years. This assumption being wrong means you're more likely to be affected by market swings.
2. Market swings won't be massively significant, for some definition of "massive", and I consider 275k -> 115k massive. Meanwhile, the Z-estimate of my house peaked at $618K this April, and is now at $561K. I would not consider that one massive.
3. That people stopping making payments because it's underwater, and not some extenuating circumstance, like losing your job and being unable to afford payments, or having a need to relocate that is unrelated to your home being underwater.
But it's #3 that really gets me. I fail to understand the "My mortgage is underwater" -> "I should stop making payments" logic jump.
I think it's because I do not see my house as investment or even an "asset". It's my HOME. I agreed to pay $338K in 2015 to have a HOME. If there was a real estate crash immediately afterwards and I was underwater, my only regret would be that I could have bought the house cheaper if I had waited a little longer. I would not in any way feel incentivized to stop making payments and walk away, because not only would that mean I would still owe the bank money, but I wouldn't have a home and my credit would be destroyed, making it hard to get another mortgage.