Partially paid, to what extent she's a threat is secondary to that. The fact remains she wasn't a productive member of society before her arrest and, according to you, she's too old to be of much use now. She was granted a second chance when her life was spared, the price was and should be to spend it in prison.
Because of the notion that the penalty should fit the crime. You don't execute someone for jaywalking but it is an appropriate one when someone engages in the wanton murder of innocent people. Some crimes are heinous enough that the forfeiture of the criminal's own life is the closest society can hope to get for justice.
Freeing her does not benefit society at all. Keeping her incarcerated when she should have been gone decades ago does give some sense of justice. She is a burden no matter what. Taxpayers had to pay for keep all these years. What is she going to do now freed? You keep pointing to her being old. Society is completely different from the 1970s. She's either going to profit off her crime with book & movie deals, which is revolting and a slap in the face of the families who are just suppose to suck it up and move on. Or, she'll continue to be a burden by collecting Social Security (which she never paid into), Medicare and other programs - which is money better spent on lower income citizens who have never participated in murder.
It changes nothing.
And once again... Moving on doesn't mean a criminal should not bear the full punishment for their crime and she only paid partial. The fact she was allowed to grow old when her victims were denied so is generosity and mercy enough.
She was a 19 year old from a middle class background who got into drugs and rebelled against her parents. She chose her path. There were many Manson members, people came and went. She was neither his hostage or under his attention 24/7, she was just one of many. Deprogramming doesn't happen overnight, she and the others stopped mimicking him once reality of their futures began to set in. That's lucidity and a sense of right and wrong, just like she knew right from wrong to be a model prisoner. Compare that to one of his true believers,
Squeaky Fromme.
According to The Trial of Charles Manson by Douglas Linder:
Van Houten dismissed three defense lawyers in succession for claiming her actions were attributable to Manson's control over her.
And in Helter Skelter by Vincent Bugliosi:
When her lawyer was asking an expert witness about the effect of LSD on judgment, Van Houten shouted that, "This is all such a big lie, I was influenced by the war in Vietnam and TV."