<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Maybe they lose value, maybe they gain. If fares go up and/or miles-for-free-ticket go down, I think that your existing miles gain in value.</font>
History is pretty clear on these two facts:
1) The cost of long-haul coach travel has go ne done, not up. If you plan appropriately, you can go to Paris, Hong Kong, Bangkok, or London cheaper now than ever before in the history of aviation.
2) Airlines over time raise the price of rewards and/or make them harder to get by placing restrictions on them.
There probably are certain circumstances where miles have become worth more (perhaps premium cabin travel has become more expensive over time), but for my flying patterns, I need to use my miles as quickly as possible - I will still be able to fly to London in coach in the Spring for $500 from KC in 5 years, but will I still be able to get a reward business class ticket for 80K or 100K in miles?