There are a number of examples I can think of where signatures really took a turn for the worse, but at least they were still alive at the time to sign still.
Ed Stevens died recently and his later signatures looked like he was trying to write through an earthquake. His hand was obviously quite shaky at that point.
Buck Leonard got really shaky as well.
Of course, Roy Campanella is an unusual case, but his post accident signatures are basically scribbles. I am not sure if he had an aid in writing (autopen) or if he had enough mobility in his arm to actually write. I seem to remember seeing photos where his arms are raised, so I am guessing he may have had enough mobility to write.
Duke Snider's signature was starting to get shaky before he died.
I'm sure there are many others..