Anyone see Mike Boyd's column at 
www.aviationplanning.com ?
Click on Hot Flash...  In this week's column Mike discusses that in nominal terms, yields have not increased since 1978.  (Evidence?  PEOPLExpress sold transcons at $99ew , and NE-Florida $69-$89ew in the 80's)  When adjusted for inflation, yields have declined 50%.
My take on this is that airlines have done a few things over the years to compensate:
1.  Increase load factor
2.  Lower costs
     a. fuel - more 73G's are much more fuel efficient that 73S's
     b. distribution - remember travel agents and commissions?
     c. service - less FA's/per flight, less food, less clubs, less FC seating
     d. technology - self-service check-in, internet purchasing, e-tickets
3.  Become more efficient
4.  "Right-size" operations - did EVV really need 737's?
5.  Use FF programs to retain high-yield traffic
Today, we are seeing the airlines take on the employee costs part of #2.  Beyond that, what else can the airlines do to continue to lower costs?  I think efficiency/technology is one answer, but I think the largest cost savings that the airlines can attain is largely done.  Prices simply have to go up, eventually.  How many other products/services can you buy for the same price as 1978?  Ok, gas may be one  
😛 , name another.