To live or not to live?

Modern medicine is, to put it simply, nothing short of miraculous.
I work in a hospital and still all these new procedures and new ways to prolong the human life has never ceased to amaze me.
But sometimes I wonder, is modern medicine really bringing us more good than harm??
I look at some of my patients sometimes, depending on all kinds of machines to survive. Getting invasive tubes inserted, getting little devices inserted to replace the heart, barely hanging on to that thin thread of life.
Sometimes, is it better to just let go, and go the way nature intended? Even ( who knows) the way God intended?
What is the point to prolong life when you are going to go through life being just a shadow of the person you were? No longer able to walk and run and jump and dance. No longer able to eat whatever you wanted to. No longer able to sit in the sun whenever you want to, to take a shower without someone there watching you, to even move in your own bed without someone there assisting you.
How horrible it must be to live life like that.
I believe we should all fight to live, but sometimes, when it is obvious who is winning the battle, would it be wiser to just let go, and leave this life with whatever dignity you have left? So that people remember you as you were, not what you had become?
This is really grey area I know, but it's just been on my mind for a while.
Feel free to comment :)