At least one of three things happens if we capped metro-area development.
1. The urban growth boundary expands
2. Real estate prices go through the roof (moreso than they do already)
3. People stop moving into Portland
I think we can agree that #3 is quite unlikely, unless something is done to make Portland far less of an urban paradise than it currently is. Do you want that? No? OK, great.
What has happened in the past is a combination of #1 and #2. Metro has slowed expansion of the growth boundary lately, and the less urban development allowed the greater the real estate bubble created. This inflation due to artificial scarcity stands in contrast to real growth in value that can arise from true neighborhood improvements, of which i was fortunate to see plenty while living in Laurelhurst in the '90s. By encouraging in-fill, the city can develop previously undervalued areas and by doing so lead to steady improvements all around for nearby homeowners.