Actually, it MAY or MAY NOT be incorrect. There is a long history to the line of thought that souls may languish long after they have paid there debt.
The Book of Enoch might be the first to suggest it. It is apocryphal, but ancient, and Jesus had read it and is mentioned in the Bible.
Also, the ninth of the fifteen promises to those who pray the Rosary is "I will deliver very promptly from Purgatory the souls devoted to my Rosary."
That promise would need not be made if the souls were automatically released.
According to Catholic doctrine, many souls will stay in Purgatory until the final judgement, which leads me to believe that they stay there because nobody prays for them. After all, we can't believe that the fires of Purgatory are not capable of doing the job.
Most of the actual workings of God and Purgatory are subjective, and the Church itself does not agree on what actually happens, only that there is a Purgatory.
Your explanation sounds good but it doesn't make sense to me. I don't think it would stand up to Aquinas.
Interesting, I have never heard that explanation before. I think I would have to disagree with it, but by no means am I an authority (obviously).
I would agree with your assessment of my explanation- it wouldn't stand up to Aquinas. I only wish I understood what he thought on the subject (and all others for that matter). Perhaps with more time and a lot more reading. . .