I think your extreme example is a straw man argument. Such an extreme shift is practically the polar opposite of what we have today.
Umm, no what do you mean? If we had a depression we would not have lots of extra homes? I think it is a rock solid argument quite the opposite of a straw man.
As I said conditions in between would cause a result somewhere in between.
It is your argument I am trying to put more succinctly. Maybe only for my own benefit as few people usually agree with me bull or bear...
Conditions change we utilise our stock differently. It is possible with no more dwellings and a population increase of 1million with bad enough economic conditions we still have a surplus stock of homes. But if underlying conditions don't change then neither will utilisation of stock just because we want it too.
Remember conditions like any of the following could change;
easy credit no more; surplus homes.
more expensive credit, interest rates rise; surplus homes.
unemployment rises; surplus homes.
sentiment changes; even this could create a surplus of homes.
wishing it were so but no underlying change in conditions; no surplus homes.
You have to have a change in conditions to move a market price in equilibrium. It is working out what a change in conditions will do to price is where the money is. Not just buying cause house prices always go up.
As you said people will not rent out their spare bedrooms. But they will if they cannot make ends meet any more, they will when they have to it is that simple, if conditions are right for it we might have 10% too many maybe 20% too many homes for the population. But only if the conditions are right for it.