"One of the great things about the double-barrel open source
software and fee-based subscription support strategy that
commercial Linuxes have is that there is very little economic
pressure on Linux shops to upgrade from one version of the platform
to another as their Linux vendor (or vendors) tweak, change,
upgrade, and improve their distros. They get to move when they feel
like it, and vendors who want to keep their customers agree to
support their older releases for a long, long time.
"This runs a bit contrary to the traditional Unix and
proprietary server space, where customers are always under pressure
to move to the latest hardware, which often requires the new
software to make use of the new hardware features..."