To retrofit an existing home or engage in new build - an open question

As many people are now doing, I’m currently improving the energy efficiency of my dwelling.

However, this made me think about the limits to what can be done with an existing structure, in my case one built a few decades ago when building standards were not as focussed on energy efficiency of the finished home as they are today.
 
As many people do, I find myself running through a list of improvements.  Increasing depth of loft insulation, check.   Replacing older windows with new lower U-value ones, check.  Evaluating different space heating solutions, check.  Each additional aspect checked off seems to have lower return on effort, time and money invested, or higher up-front cost involved.
 
There are many factors that determine what the list looks like for each person’s dwelling, and those affect the relative costs and benefits of acting on each item in the list.  However, inevitably, at some point, it would become more cost effective overall to stop refurbishing the existing dwelling and instead do a new build from scratch. 
 
This is really an open question, because, again, a lot depends on where that new build would be, what it would replace, to what extent it would be like-for-like in terms of the various elements of what J S Mill would call the “utility” it would provide to the occupiers, what would happen to the older dwelling, how it would be used, maintained or improved by any new occupiers, and so on.
 
The bottom line is that, for most dwelling occupiers, the energy efficiency and other sustainable living criteria are almost certainly not the main factors in deciding whether to continue to refit an existing dwelling or build a new one.  I suspect (and hope) this calculus will alter somewhat as sustainable and net zero carbon living will become more of a mainstream “thing” through to 2050.