The reality is that demand is always going to far outstrip supply in certain areas. You can only cram so many houses into a place like San Franciso, even if you raze the city to the ground and replace it with some yimby paradise that houses 25% more people. The extra supply might lower costs to some degree in the short term but the induced demand will ensure they stay above what someone on an average salary can afford. Everyone seems to understand this when the topic is freeway expansion, but not with housing.
The actual solution for housing costs is a lot more complicated and multi-pronged, but that doesn't fit neatly on a bumper sticker. There actually is a ton of affordable housing in the US, but it's mostly in areas that people in influential industries like tech and media tend to sneer at, so it gets dismissed; and suggesting someone move there is practically treated as a human rights abuse.
Dramatically increasing housing density does have adverse affects for existing residents. It's fine to say the tradeoffs are worth it, I'm not saying it's never the answer, but even acknowledging that it's a tradeoff usually gets met with dismissive insults from the yimby crowd.