It would have been a tragedy if the gentleman were inadvertently euthanized, despite his "persistent vegetative state of 23 years," when in reality, apparently his cognitive state would ultimately "correct" itself.
However, I would consider Euthanasia as an option for any patient requesting it, were it legal, and they patient met the criteria, including being in "sound mind" or being represented by an advocate and the state, if not.
The example you cited is a bit of an anomaly and human medicine is rife with it. However, it is also rife with suffering in which the sufferer does not have the luxury of lying in coma, but must agonize horrifically. Even with morphine, the priceless gift of life is compromised too severely with the pain of the pathology, of which there are thousands.
How about a Human Being who has contracted rabies? It's even more lethal than the Ebola virus, with a 99% fatality rate. Should the searing fever, madness, intense anxiety, agonizing muscle spasms be treated palliatively, as the victim's central nervous system is ravaged and their brain seared by a fever that literally destroys it? Or after a certain point, should the victim be spared the ordeal that awaits them, as a fever ravages their brain irreversibly.