Maybe I'm wrong, but my assumption is the difference between "standard home" and "standard home with solar + other easy wins" (such as tearing out your lawn in SoCal, where I live) is overall "better" than going from those easy wins to a Zero Energy home. I would guess it's also location dependent though.
Under that assumption, I would have a hard time justifying tearing down an existing home to build a Zero Energy home, as opposed to just implementing those easy efficiency wins and investing the money saved in other environmentally friendly pursuits - perhaps even helping a friend to finance similar changes to their home. Or, letting someone else buy the already built home and build somewhere I don't have to tear down an existing structure.
Of course not everyone would think this way, and people are free to do what they wish with their money, but it's how I weigh the options.
Under that assumption, I would have a hard time justifying tearing down an existing home to build a Zero Energy home, as opposed to just implementing those easy efficiency wins and investing the money saved in other environmentally friendly pursuits - perhaps even helping a friend to finance similar changes to their home. Or, letting someone else buy the already built home and build somewhere I don't have to tear down an existing structure.
Of course not everyone would think this way, and people are free to do what they wish with their money, but it's how I weigh the options.