It's a pretty simple argument: As long as the remakes are still successful then there is still demand for them. Purists and people who aren't fans of the series can look at the number of remakes and make snap judgements but ultimately this doesn't matter. If the fans want more then it is completely reasonable to give them more.
The Final Fantasy franchise has provided millions of gamers with countless hours of fun, frustration, humor, and wonder. Its specific game play is full of quirks and limitations which fans of the franchise actually love, finding comfort, satisfaction, and joy in the familiar. I can see no reason to ever end this beloved series; I look forward to Final Fantasy MMLV!
Consumer desire is what drives every market in capitalist economies, and thus if the consumers are paying for additional remakes of Final Fantasy, then there will never be "enough". As long as the demand is there, and as long as developers continue make money on this demand, these remakes will continue to be made. Additionally, as someone who is not part of the gaming world, I don't feel that it is my place to determine what is "enough" for those who do have an interest in this genre of entertainment.
As long as there are fans who are loving the series, why not keep making remakes of the game? Keep the fans happy and keep making money for yourselves! It wouldn't make sense to just stop making remakes if people are still liking it and still get very excited when a new remake comes out. Keep it going!