What? We can't do 5+1 everywhere because first we need to litter every last bit of countryside with failed sprawling towns that were swallowed by their own maintenance costs?
That doesn't seem workable.
That doesn't seem workable.
The suburbs take a huge amount of land. If everyone lived in NYC-level of density, we could all fit in New Jersey. If we all lived in middle-missing housing level density, that amount of land is still far less than the total area of all the suburbs.
Upzoning does not mean you automatically get a building at maximum allowed density, it means you increase what is allowed and let the market decide what makes sense where. It would play out exactly as you describe.