Portland has a problem with dense development. The problem is that we are too willing to reject it, whether because we're enamored with what we have or afraid of what is new. That mix of complacency and xenophobia will in my view doom efforts to build and sustain a livable city if it goes unchecked.
One problem is suburbs. When we don't accommodate people living in the city, we force them into suburbs. By their nature such areas are vastly more car-centric, and its citizens car-dependent. Disproportionate population growth there will, through democracy, lead to a more car-centric public policy. Highways instead of streetcars, more space devoted to parking instead of retail or other human use. We can't force most of the metro population into an environment antithetical to livability without inviting blowback.
Another problem is economics. What happens when you increase the desirability of something, but not its supply? Prices go up. And in Portland, housing prices go up a lot. Despite the bubble, housing prices have increased more than 60% since 2000 as measured by the Case-Shiller index. How many households have seen their income grow by 60% over this time?
Inflationary housing prices mean the city will be accessible only to an ever smaller, wealthier class. That compounds the public policy problem (why should poorer suburbanites accept policy dictates that cater to urban elites?), and it reduces Portland's diversity. Cities thrive on diversity and the interplay of different ideas that come from different backgrounds and experiences. That dynamism creates jobs. Restricting housing in the city will push out not only people, but creativity and job creation. How livable is a city without those?
Here is a litmus test for Portlanders. Take the number of years you've been living in your house and imagine someone born that many years after you. Suppose this hypothetical person grew up in a similar environment as you and made similar choices about education, family, and career. Would they now have the same ability to move into a city neighborhood, not necessarily the same neighborhood but one with comparable amenities, as you did years ago? Extending the idea, I'd say a community which by design bars our children from doing and living as we do cannot be called sustainable.
It isn't enough for livability to be a good idea, an affectation that we put on like a fancy hat. To mean something in the real world it has to be able to grow.