Final Fantasy sells a ton, it will most likely continue to do so and thus it will continue to be made.
Sales first, fans second. That is the order of business, not the other way around. If a game sucks to the diehard fans, but sells more to the vast majority, the company doesn't care they will continue to make what sells. FF whether it improves or worsens, will most likely continue to sell and therefore will continue.
Although I have to say FFXII definately looks to be the best PS2 FF, not that that is saying much. I have played 4-10 and I like them all, they are all good, some better than others but all good. I am sure I can name something I like and dislike in each, it happens and will continue. You can not please 100% of the fans no matter how good the game is. (For example many liked FFX-2 and I absolutely hated it, but I really liked FFX).
So, in answer to your question, no I do not see the series ending any time soon. Which is a good thing, as there has only been one FF I didnt like and that was a spin off more than a main game.
