A generation doesn't end until all of its consoles are off the market. We may switch our terminology to refer to newer devices as "current-gen", and the older ones "last-gen", but multiple hardware generations can run concurrently. Actually, gen 6 is still ongoing right now since the PS2 is still being sold. Three generations at once. Probably for not much longer.
Edit: For further clarification, I'll try to use living beings as an analogy. Multiple generations of humans can live at the same time. The life of an older person doesn't end when a younger one is born, though there is a point in time after which a person must be considered a part of a new generation. New generations bring change to the table, but a generation ultimately stops existing when its members cease to exist. Relevancy is a seperate issue.
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