I don't think it is any developer's responsibility to build affordable or low income housing. Developers are not social services agencies, they are for profit businesses that build the product that is selling. There is no shortage of people who rent high end apartments or buy high end condos, townhomes and houses, that is why they are building them.  

Unless a developer's business model is building affordable housing, what Seattle and other misguided cities's political hacks are doing is transferring what was once the government's responsibility to private developers.  That just does not work.  

The more regulations and red tape and expense you add on to a developers bottom line the less they will build.  How many units have we actually lost because developers are staying away? Do we expect these business, who employ thousands of people to just operate at a net loss? 2.5 percent of the total  population gets a sweetheart deal on their housing while the majority pay through the nose?