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when support for an OS dies, the hardware manufacturers will begin to stop supporting the old OS usually prior to the end of the OS [why bother creating drivers for an OS that has say 3 months to live].
You also have software developers who may stop any support such as Google Chrome [guarantee they will do that!], Firefox, etc. AV/AM software will continue for the current version but no new versions.
Plugins such as Java [yeesh!] and Flash could be stopped as well.