Banks buy Airmiles, Hotel Points at dirt cheap rate say approx at value of Rs. 0.2-0.25 per point. They offer for Magnus and Reserve at approx Rs.0.8 ratio in which they make profit only.
Coming to Banks and Airlines they offer banks at that cheap rates because usually Airlines and Hotels will have rooms vacant and seats vacant while flying, so they want to fill it up. This inventory is called "Distressed Inventory" in their terms. So win win for Airlines and Hotels, but this is not viable on large scale. Usually Super premium cards are of 2% only of the lot the scale of users are less.
When banks run out points or Hotels/Airlines increases the value of points then devaluation happens. So lets hope the party lasts longer and enjoy the party.