It is important to remember that at that time diagnosis was very primitive. Decline could simply be wasting away from almost anything. TB was known although the cause was not identified until 1882 by Robert Koch, but someone with symptoms of lung disease could actually have been suffering from something else. There is absolutely no way that diseases could be more specifically described for the simple reason that many, if not most, had not been identified or even named. It probably was TB but even if the certificate said that there is no proof it was because no one knew how to prove it. A similar expression is "Inanition" = "failure to thrive" applied mostly to babies - why were they "failing to thrive". Your guess is as good as anyone else's. Accurate diagnosis of many things known today was not possible often well into the 20th century. It had nothing to do with competence - just with the state of knowledge.