I remember when wind turbines at sea were first being installed - the cost was insane . But economics of scale and refined technology makes them a good investment. I do wonder when the next leap in PV efficiency will occur ? ( what are we at 20% in real world situation ? ) . Hydrogen might have its day for replacing the gas combi , but infrastructure and production costs prohibit that currently.
Agree with @ProDave most old housing stock is crap . I really struggle to get my rentals to a ‘C’ and that’s after spending considerable sums ( new double glazing , new roof with insulation) but only because the law will / does force me . Private owners in older housing have no real incentive. The cost of work against the saving is negligible. It’s certainly a tricky one !