If a Seattle resident would have been willing to buy a presale (to ultimately live in), but it is instead bought by a foreign investor, then the resident has a

If a Seattle resident would have been willing to buy a presale (to ultimately live in), but it is instead bought by a foreign investor, then the resident has a smaller pool of housing to choose from. Only if the foreign investor holds on to the condo in question. If they sell the unit before construction is finished (which is the most common scenario), there are just as many units of housing available for local residents. The only supply being reduced is the supply of pieces of paper which entitle people to purchase units which don't exist yet.