To some degree they do, at least with schemes that give higher fares more than 100% of miles flown. Many of these also count those bonus miles toward status.

Tying awards directly to revenue and fully to scale (e.g. $2,000 spent gets twice the benefits of $1,000) could cause a revolt with companies, which have fumed for many years that the programs add a big cost to them for a small benefit for employees. There's often a suspicion in big companies that some employees game the system, waiting to book late because they know that expensive Y ticket will probably get upgraded and have more miles. Large companies have tried before to get airlines to offer non-mileage-earning fares for a % discount.

I think the airlines really crossed over into pyramid-land with all the non-flying sales of miles in recent years.

I think the crunch will really hit when baby boomers retire (62 in 2008, 65 in 2011 and after). Lots of people who had to hoard miles during their careers will want to use them to travel in retirement, and that'll exacerbate current imbalances.