Not sure I agree with that. Yes you need to factor in the benefit of flexibility on an avios ticket, but that doesn't mean you should compare it to the cost of a flexible cash fare, which is usually stupidly expensive i.e. several thousand pounds difference. I've never in my life bought a full flex cash fare and have never had to cancel/completely throw away a long-haul ticket, and of course also have insurance which would cover most situations where I would need to.

The maths is all subjective but you do need to be realistic about what you might pay in practice rather than in theory.