I'm not a huge FF fan, and not a fanboy, but to answer the actual question, Is FF getting old?
Literally: yes. The first FF game came out, what, 20+ years ago?
Figuratively: No. I find that each game I play has enough new stuff to add novelty, while building upon concepts, creatures, characterizations, and general 'feel' that it is still somewhat familiar without being a stale rerun.
Are they the best games EVAR?!? No. There's no such thing as a perfect game, and never will be. They're fun, and yes, more mainstream than some RPG's--if it takes even the most mediocre FF to get accepted, to allow me to buy Persona or Nocturne or La Pucelle, then so be it.
But neither are they the worst games ever. I just finished FFx-2 at 71% percent about about 65 hours. I feel that I more than got my money's worth. It wasn't a super deep game, and of course there are things I'd change if I had my druthers, but I enjoyed it as I was playing it. I don't know if I'll replay it or do the New Game, since I've got other games I haven't beaten yet, but I'm glad they gave me the option of doing so.
Good but not great, every game is someone's favorite, always room for improvement, but still possesses some novelty.
And FFT [the original, on the PS1] is probably my favorite console game of all time, or at least in the top 5.