Speaking as someone who spent a good part of my life renovating homes, I can tell you that there is not much of an "upper limit" to the energy efficiency of an older home. If you're already planning on renovating a home, there is an incremental $25-$50k energy efficiency improvements that you can make to turn the house into a nearly net-zero energy home.
The issue is that the young couple who do renovate their home have a far better ROI on updating the kitchen + bathroom than the efficiency of the house.
As half of a young couple that's at the cusp of starting a remodel (focusing as you point out on bathrooms and the kitchen), I'd be very interested in your thoughts on what's possible in terms of efficiency.
We're replacing the insulation in the attic and getting mini-split based cooling/heating. We also replaced the windows and briefly considered blown-in insulation for the walls, but weren't convinced that it would be worthwhile.
The issue is that the young couple who do renovate their home have a far better ROI on updating the kitchen + bathroom than the efficiency of the house.