This current tendancy to replace comparatively new airport people movers after just 20 years of operation is absurd. Was the New York Subway completely replaced by a new system in the 1930s? No. Was the San Fransisco Bay Area Rapid Transit (BART) system rebuilt from the ground up in the 1990s? No. So it seems to me to make absolutely no sense to treat airport people movers as though they were private automobiles and replace them after a few years in service, with a newer and sleeker looking model.
The argument that they handle an excessive amount of traffic is easily quashed by the fact that though this is the case, they were designed to handle said traffic, and never do they face loads more severe than what you find on most subway systems.
What should be done is the existing systems should be refurbished and modernized. The rolling stock probably does not need replacement for reasons other than aesthetics, but could be freshened up, with perhaps upgraded doors, windows, paint schemes etc.
However, ultimately, the cause for all of these system replacements are probably the corrupt officials of transit system manufacturers. Having done a lot of research into the mass transit industry, I can say there is an incredible amount of idiocy, in particular, there is one rather large Canadian firm that also manufactures Regional Jets that has a tendancy to produce a poor quality product (but they are the largest railway manufacturer in the world, having bought out so many other companies, that they are virtually guaranteed to get a huge number of the orders).