Cannot speak for the poster, but the big current issue is that the value of residential units has gone from being strongly linked to wages to residences being an extremely valuable chip for playing financial games. Residences, especially inherently valuable ones like NYC apartments, have sufficient demand from financial game players that wage earners are barely able to keep up. Exactly how things break down is not clear, but there is this idea that demand for residences for financial purposes is competing aggressively with people who want homes.

This is a well studied idea, the percentage of units sitting empty is a round error in the total supply of housing. At the start of the pandemic there were a paltry 4000 vacant condos. For reference there are 2.3 million apartments in the city.

That is not directly related. People and institutions holding residential units for investment purposes usually rent them out.

Then they're on the rental market and don't contribute to a shortage in supply.

But how many vacant apartments were there (or how many total condos)?

The vacancy rate of apartments is just at or under 4.5% which is considered a very tight rental market. I don't have the total condo numbers readily at hand.

Because I grew up in Brooklyn and Queens and saw what all of the new development did to the demand. The street that my grandmother used to rent an apartment for in the early 2000s for under $1k/month now only has units that go for $5K/month.

Here's greenpoint as an example: https://www.zumper.com/rent-research/new-york-ny/greenpoint. Same goes for LIC.

As a Californian, if the demand is there, it will eventually bid up trash fire apartments to 2k a month, even if they're still colocated with violence and bad schools, you don't need to induce any demand with development.

Maybe because Greenpoint and LIC became a lot more in-demand in those 20 years and development followed the demand?

You have that backwards. Bloomberg rezoned the queens and brooklyn waterfront for his real estate investor buddies, which gentrified the shit out of these neighborhoods and brought in a ton of people that would have never dared to cross the east river.

So they allowed housing to be built to meet the demand of living on the waterfront.