I wouldn't generalize from one example. Many flights generate more distance miles than payment card miles. You can fly from MIA-SEA-MIA (about 5,400 miles, roundtrip) for under $500. As airfares rise the proportion earned by flying (distance) rather than paying (dollars) will be reduced. This isn't any type of program devaluation.
In a sense, it is a type of devaluation.
As prices rise (not just airfare, but general inflation), the number of miles needed for awards have to rise as well. For zone-based awards, it doesn't happen smoothly; instead it happens in chunks. This has to happen -- if prices double over a 10 year span, they can't give you a free flight for spending the same number of nominal dollars on their credit card, since that number of dollars is worth half as much. It has to be inflation-adjusted.
As a result, if they stick to "1 mile per mile flown", those miles flown become worth half as much. What they should do is increase the miles earning rate as they increase the number of miles in the award chart, but this never happens.
Revenue-based models don't suffer from this problem.