I don't know where you got that. Affordability downtown isn't really what is keeping people away (and it is not like rental buildings have lots of vacancies, nor are close to downtown condos, towhomes, and SFHs difficult to sell), but retail and restaurant choices have been decimated over the last decade, it isn't thriving from the point of view of someone going there to do things.
The homeless factor can't be understated in Seattle, especially with all the encampments downtown. I worked at 3rd and Pine McDonalds in the mid-90s, and for as bad as it was back then, it is ten times worse today.
So at least as far as its peers in the PNW, Seattle isn't very much of an outlier. Well, unless we can compare to other Seattle-area cities like Bellevue, which can simply push most of the homeless problem onto Seattle.
The city is trying to move people into housing but there's a lot of mental health and addiction problems that make many people resist the outreach support.
If I'm reading it right, the problem you're calling out is that it is unaffordable to live in a downtown metro after it reaches a certain density. People still think they can hack together solutions by subsidizing housing, rent control, applying grants, etc but I'm more of the mind that there's probably a max size to a city, where once you hit a threshold it either takes a nosedive into poverty or becomes so unsustainably expensive that it chases out what desirable things may have existed there in the first place.