Thing is we are gaining on the Sci Fi side now. We have metals that can actually bond with bone so that it becomes an integral part of the human body. I know because I have some implanted in myself, and would not let the Doctor do it until he explained to me how it would not just rip out if I got hit again.
Stem cell research has grown by leaps and bounds, as well as cloning. This does not equate to a Space Marine, but if you took Jules Verne while he was alive and let him witness these things he would have been amazed.
I agree that it is far fetched and some of it is poorly thought out ( a fused rib cage would not work ) but I see in the future that things will change. Hell, look at just 50 years ago. If you would have told a doctor " What I want you to do is put a drop of fluid in my eyes and then set your phaser to stun and blast my eyeball with it, and when you are done I will have near perfect vision! " you would have been on your way to the mental ward ... because that shit is just crazy. (Thank GOD I actually have good vision and it is the wife that wants Lasic Surgery done, that shit scares the fuck out of me)