The people in the old house had windows that had a bonus view for a while. Now the property's being developed and their bonus view just becomes a wall. If they wanted to maintain line of sight they should have purchased this property next to theirs.
It'd be a crazy world where you can't build on your property because you would block the window of somebody else's property
Do local govts/authorities not block people from blocking others' windows?
Around here you have to be a certain distance away from other windows, have your roof at a particular angle to make sure sunlight still reaches and a bunch of other stuff prior to approval... and even then neighbors are still free to block construction if it's impacting their light