This is easy.

As the liabilities of billions and (soon-to-be) trillions of FF miles continue to build faster than they are being used, the airlines will simply inflate/restrict them away. We're already seeing ridiculously few seats available on popular routes and fares that are non-upgradable.

Being able to transfer them for non-airline awards will still be possible like it is today, but it allows you to see what the true value of a mile is - and it's not much at all, less than a penny.

I could buy the "2 cent per mile" argument if you could actually redeem them 100% of the time, like you can the non-airline awards which have much fewer restrictions. Imagine trying to redeem a coupon at the store and being told that too many people have been saving money so YOU have to pay full price.