IIRC the boom started around the grunge era(30 years ago) when people in east and central cities started to realize Seattle sucked less than their frozen wastelands. I don't think it has slowed down since. All cities have booms and busts, but that doesn't mean you plan for the busts.

You fix the added demand with zoning and a comprehensive regional growth strategy. Not building freeways to service the demand you've have for decades is just insane.

I don't think there has been a single new freeway added since I can remember(early 80s). There is still a huge reliance on surface streets and 2 lane roads to get to the bedroom communities on the outskirts. Not adding lanes to, say, hwy 18, hasn't done a damn thing to limit growth in the east side. It has turned the region into a giant clusterfuck around rush hour though.

Again, Seattle has been a boom town ever since it was founded. The grunge era was actually kind of a bust (if you remember, unemployment was a bit high and housing prices were stagnant in the early 90s), but the start of maybe what we can consider the current boom?

Seattle is a boom town, it doesn’t just have a conventional boom or bust cycle. All west coast big cities (Vancouver BC, Portland, SF, LA, San Diego) have a similar rapid growth profile with similar problems. Note that none of these cities have been able to solve them adequately, and we ran out of space to build new freeways decades ago, so that solution is never happening, even if anyone thought it would solve the problem at all in the first place.

A city like SLC, which has room to add freeways along with republican run government to add them, hasn’t been able to fix its traffic problems either. In fact, they just get worse as the new freeways encourage more sprawl.

So an impractical liberal city and an impractical republican city both chose unsustainable paths and ran into trouble?

I think you need to combine realistic freeway growth with other growth limits to restrict sprawl or, you're right, you become SLC/PHX/etc.