2. It's not always profitable or practical to tear down a building. Tearing down a 3-flat and replacing it with a 4-flat, for example, would not be economically feasible even if it were allowed. Replacing a 3-flat with a 12-story building, on the other hand, would result in a much more productive use of the land, but it's typically not possible to upzone a parcel to that extent.
https://www.nytimes.com/interactive/2016/05/19/upshot/forty-...
I think it is clear that zoning is a primary cause of NYC's housing problems, but I just don't see any evidence that height restrictions make up much of the problem (remember, the upthread claim I took issue with is "height limits are the only reason there's a shortage of housing").
Anecdotally, a lot of buildings for sale (in Brooklyn, at least) list both the number of apartments and the max it is zoned for. This is relevant because it isn't actually that expensive the reno a 3-apartment into a 4-apartment (no need to tear it down), so zoning ends up being the primary roadblock for people who want to do that.