I agree - although Square hasn't yet announced any plans to make FF XIII and beyond, they haven't (to my knowledge, at least) made any plans to end the series. I'm sure in another year once FFX-2 and FFXI have blown over, and Advent Children and FFXII are out or nearing completion, we'll hear about their future plans. Personally, I'm always amazed at how fast they churn FFs out.
As Squall2 said, as long as FF games remain lucrative, I doubt they'd stop. They are a business, and the series will only truly end when we stop paying for it. So I wouldn't worry.