We cannot do everything. Resources are always finite on a finite planet. I wasn't saying we should only do things with short payback times. But a 75-100 year payback time seemed a bit steep.
Using scarce capital on replacing G fridge-freezers by A+++, and other measures like it, would yield CO2 returns sometimes 10-100 times greater. Ditto replacing controls on even gas condensing boilers, etc, etc.
Long term, we shouldn't be heating buildings via gas boilers, as I tried to imply in the article, in part via a reference to Denmark. I hope we're not doing so in 2100 although I won't be around to know either way.