In the end (spoilers)
In the end Miss Justineau is in the same situation that Melanie was in the start of the movie. Locked in a cell, unable to interact with others in anything but sight and speech. I've seen many comments that she would not have been able to take that. Maybe that would be true in the long run. But it's not really different from how Mellanie lived most her life. And Miss Justineau was complicit in that.
There is no such thing as too much time