The FF series will end when everyone who worked or was associated with the original game is either dead, dying, or incapacitated, and unable to contribute in any way, shape, or form. This'll take the series to the 20s, but not much further, depending on the popularity of the series.

Or the second generation of FF makers will be able to take the mantle of their predecessors, do them proud, and create a string of games even greater than the originals, ushering in a golden era (well, even MORE golden) for RPGs in general. And it'll thusly establish FF as a never ending series until video games are no more.

Or not.