I don't believe the main issue is knowing whether you are going to have to move soon. The main issue is buying a house and then shortly thereafter being laid off/fired/company went under/etc" but with jobs available from somewhere not close enough for the current house location to be viable. If its a couple years or so and house prices go up, its not too harsh of a financial impact to sell and move. If houses are falling, moving for job opportunities are that much less amenable.

But the new house that you will have to buy will also be cheaper. On average, you are no worse off.

What new house are you going to buy if you don't have the cash to pay off the underwater mortgage to sell your current house?

In the UK people have historically (within recent decades) moved liberally and often even if they've just purchased because house prices have just gone up and up and rates down and down

Maybe it comes up in every thread because it's an actual issue. A lot of people move after they buy a house.