This game can continue forever if banks increase cards' earning rates to 1.5 or 2 miles per dollar and keep increasing it in step with award chart inflation.
BAnks buy the miles from airlines, so for this to happen they have to pay half which airlines won't do, or pay twice, which banks won't want to do. In the end, they'll effectively be giving away 2% cashback, in which case they might as well just tempt people with 2% cashback cards.
Now there are smoke and mirrors where they can fool consumers into thinking they're getting over 2c of value per $1 spent (and some get that value) but most never see that return from miles.