I get the sense that your implying FFXI would go F2P before FFXIV ever would.. I would say although likely, thats not exactly true. FFXIV could be a complete piece of garbage that SE basically abandons (as well as its player base) and then puts all their resources back into FFXI... A game that is soon to get another expansion and is still gaining brand new subs (and old returning subs) every day.
However, I also think that no FF mmo, unless built this way from the get go (But why??)... would EVER be reduced to F2P.