More to the point: once bodily health is damaged, it's not coming back.
For example: Lindsay Lohan will also live much shorter than she could have (in an alternate world), even if she lives a perfect life with zero relapses from her last substance abuse. The same goes for any other person with a cumulative history that may have reached or passed a critical tipping point of irreversible damage (which includes say, type 2 diabetes).
To be fair, I though Fisher was actually older, like Harrison Ford (currently 74). That's why I said "borrowed time" earlier when referring to the entire cast of the original Star Wars, not just Fisher.
But in the end, there are two things to keep in mind: one: the average cohort (by year of birth) lifespan is skewed towards the centennial outliers due it being a linear measurement, meaning that the average and median without those outliers are actually significantly lower, which is a good thing to keep in mind when thinking about retirement. Two: once the heart, liver, or brain receive significant damage, there is no going back. There are no effective treatments, there is no recovery, there is only some additional time before the end. But at least that is a dignified one, and it can still give considerable time. That, if possible, is what you should be hoping for in this case, not a magical recovery.
I get that people want to hold on to magical beliefs and wish for the best possible world to happen, but unfortunately that's not how life works. And personally, I believe that such expectations do more harm to a person than they would do good. If she recovers to some degree, that would be truly marvelous. I'll spare you the realistic version. But it would be equally unreasonable, in my opinion, to sustain a delusional belief that there is some 'magical healing' available. We'll see what happens, but mostly she is playing for more time right now. I do hope she gets that.