so they have character
The problem is that so much of the “character” that people want to preserve in cities is low density housing. If you want sustainability, that’s not it.
“Low density” housing is not all too common in nyc. Go out to queens, sure. But I’m talking mid rise buildings, hundreds of units.
When you’re talking about areas that are already very dense, then building up is the only rational way to go. (I’d prefer building down, but that’s more expensive.) I’ve seen facades preserved while higher, more modern buildings were built in the same footprint, and I think that’s a good compromise on “character” versus density. But when it comes to 2- and 3-flat buildings with fenced in yards, bulldoze 'em and put in high density.