pixie, I personally know a person who was told by his doctors that he had 4 months to live at most, he is still going strong 4 years later!
No doctor can accurately predict the life span of a seriously ill patient in all cases, mistakes can and do occur. Of course if this becomes law we will never hear about them.
There are cases where a patient has been diagnosed with 'locked in syndrome' (supposedly always fatal) only for them to recover and tell of how they could hear people around their bed talking about euthanasia for them but being unable to communicate in any way.