Housing is the big one mentioned, and it does not allow for any further economies of scale than already exist. Rent rises based on the prevailing salary of the area -- house prices as well, since they represent the years of rental income. It's not even necessarily a supply issue, since Seattle has more empty houses than it has homeless people! Prices simply rise to whatever the market can bear. So under the current system, any absolute increase in money will likely simply be swallowed by landlords.

Housing is a normal economic market that is responsive to changes on both the demand and supply side. The major problem with housing in this country is that restrictive policies have put a damper on housing supply so that it is not able to keep up with demand. Seattle has a normal amount of vacancies in it. It could have 2X the housing (and thus have a lot more people living in Seattle paying lower rents) and still have the same percentage vacancy. It's not a real or valid argument to say "look there's more vacancies than homeless people, so supply must be fine".

A "normal number" of housing vacancies would only make sense if everyone was housed. Housing isn't a voluntary good, where you'll expect some unsold stock and you'll expect not everyone to buy one -- everyone needs a place to live, and will spend as much as they have to in order to get housing of minimal quality. The entire concept of a "normal" number of vacancies doesn't make sense here.

There absolutely is a "normal" amount of vacancies because it takes time for a house to sell, for someone to move out and then someone else to move in, for a new lease to be signed, etc. It's exactly the same dynamic with employment; there is a "normal" amount of a unemployment (a low single digit number) that is impossible to improve past, simply because it takes time to find a job. Housing is no different.

Also, most of the homeless are unhoused because they either can't afford a home or they have mental illness/drug addiction issues that makes them incapable of earning money in order to be able to afford a house. It doesn't matter if housing is vacant if you don't have the means to pay the rent, or if they won't even consider you for a lease anyway because you don't have a reliable source of income.

The implication in his argument was that prices will increase in general.

My argument is that we don't know what the overall effect will be.

Edit: In other words, if housing increases by $10 but the cost of other stuff decreases by $20 then you still come out ahead.

You're right of course, but also way more optimistic than me.