A currency is only good to the extent that it holds its value over time and you are confident you can convert it when you wish to.
Airlines seem to have an increased propensity to fiddle with their schemes - be it the conversion ratios or via other mechanisms which make exchange difficult.
A local coffee shop stamps a card every time I buy a coffee. Buy 10, get 1 free. Its simple. I don't mind hanging onto stamped up cards - I have confidence in their value. I can always get my free coffee. I choose this coffee shop over others that charge similar prices and don't have a card. Thing is, even if coffee doubled in price, the scheme would still work. Yep, they'd take a one off hit in respect of the value of cards in circulation. But no need to undermine the scheme.
I suppose airlines will say they are on lower margins these days. Anyway, my confidence in FF miles is somewhat dented. I don't value them as much when they are given away, and I don't want to be sitting on them once I've got them.
So yes, I agree to some extent with the OP, though its still early days.