> If something actually saves money then it doesn't require a subsidy because people would be doing it regardless.
…assuming people are good at math.
And given that most people probably don't have a bunch of cash just sitting there doing nothing, they will have to take out a loan and most folks probably don't like going into debt even though 'the math says' it's a good ROI.
The idea that 'ROI good, therefore people will do it' is the 'spherical cow' of economics. In reality there are all sorts of other motivations for human economic actions: