But it isn't. You think all those SFH lots are just going overnight become a duplex? Why would the retired lady who walks her dogs and tends the garden care about building a duplex?

Now when her kids inherit the home & sell it to a developer, sure it'll happen then. But that is a slow, generational process.

Yeah exactly, it's a nice slow process that wouldn't piss people off by having too much rapid change.

I think the point is that it's far too slow to ever make a difference. Is there an existing city which has ever successfully "graduated" through all these levels?

I think you need to meet some of the NIMBYs where I live.