Believe me, they are still going to build because there is still money to be made. Besides, rent control does not apply to new construction.
In addition, the City of Portland is in the process of trying to approve the Residential Infill Project that works on FAR and allows tri's and quad's that used to be allowed but are now illegal.
Portland is seeing modest homes torn down in favor of modern McMansions, so the city has put a cap on the square footage and is legalizing missing middle housing types. Should pass this year.
And to anyone who thinks there isn't a housing crisis here, you're just a dick. $400,000 homes should not be the ideal and considered approachable for the average would-be homeowner. $400,000 buys a fixer upper in some near-in neighborhoods that wouldn't even pass inspection for a COO. Which means you probably can't get traditional financing.
The median home value here is $425k, but the supply is severely restricted and prices are driven way up. $425k would be $2349/mo inclusive of taxes for a pretty crappy house. For this to be affordable, the household income of the buyer would need to be $7830/mo, or $93,960 per year. The average household income in Portland is $86,000.
However, since we are so under-built in low-income units AND high income units, low-income people, middle income people, and high income people are competing for what would otherwise be middle income housing. As a result, poor people and people of color are being pushed to the suburbs. In Portland, the burbs are cheaper and more diverse economically and racially than the central city, which poses its own issues.
Edit:
Here's a 584 sq. ft. shack 5 miles from downtown for $250k