The 2.8 version remains for multiple reasons but the biggest reason is the older 2.8 is compatible with older versions of Apple's OS. Skype 2.8 runs on Mac OS X v10.3+ on Intel or PowerPC. Skype 5.0+ requires Mac OS X v10.5+ on Intel only.
Those who were eligible to upgrade from 2.8 to 5.0 were given notices every time they logged in to upgrade. Your choices were "Upgrade Now" or "Later". You had to go into the settings and disable the update check. You could always post-pone or disable the upgrade, but you also can't use certain features in the old version; hence "forced".
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