Excellent observation! And I believe you are right. Not one soul will leave Purgatory, nor would they even dream of wanting to, until they are "pure" enough to stand unashamed before God. There is a parable to that effect "until the last farthing is paid."
However, there is a notion that souls, even though they are purifed, will languish in Purgatory until they are brought to God's attention through prayer.
"However, there is a notion that souls, even though they are purifed, will languish in Purgatory until they are brought to God's attention through prayer."
First, isn't God omniscient? Does He need anyone to "bring something to his attention"?
Second, say someone is in purgatory, and they have been fully "purified" through suffering. If no one is praying for them, will they continue to languish there? Will God allow them to continue to suffer, although completely purified? Doesn't sound like the righteous God I know. Then again, the God I know did all the suffering necessary, and now and forever considers me and all other true believers justified, purified, clean, and righteous.