Fascinating question, as it illustrates so much about the, well, flaws in dental education. I have several answers:
-Furcation, furcation, furcation. Perhaps there was bone loss exposing the furcation enough where the prof saw that long term prognosis was poor. For many reasons, a surprising number of them having to do with the root canal system and the accessory canals that are pervasive in molars, periodontal treatment is less successful in furcations than anywhere else on a tooth.
-The overall treatent plan. Would this #15 be asked to stand as a bridge abutment? Single crowns are one thing, but basing a bridge on a questionable tooth is a far bigger risk. Or are implants, or removable, planned?
-There is, however, a wide range of, shall we say, aggressiveness in the decision of when to extract a periodontally involved tooth. My partner and I have taken a very non-aggressive approach to this over the years, as long as there is no demonstrable health risk to our patient and no major treatment plan all depending on such a tooth. Most of our periodontists work with us this way too. I've never experienced any real problems from this approach. Yes, perio is a site-specific disease, and one site can affect others, at least in theory. But we get more aggressive in treatment if the disease is aggressive, and this is relatively rare. It's all about the biofilms and their control anyway...
-What I find upsetting is your prof's attitude towards learning. Not that s/he is speaking to us directly here, but from what you say, there seem to be barriers to asking questions. This is unacceptable! On their part, not yours, I mean. If I was running a dental school, which no one would ever let me do because of all the things I'd screw up, but if I was, one of the things I'd get right is to have a universal signal that students could use when they had a question that they'd rather not ask in front of the patient, which is often the case.
Here's how it would work: when any student gave that signal, the prof and student would go off, perhaps to "get supplies" or some such, and the student could ask their question with absolutely no chance of the professor denigrating them for asking it. All questions fair, no questions considered "stupid" or any such thing. Patience and respect the order of the day.
Consider the advantages. One, students would learn a heck of a lot more from clinical faculty, as there would be no imtimidation as you describe above. Two, students would be trained, day in and day out, to answer patient's questions in the same manner. As it is, arrogance and high-handedness promotes arrogance and high-handedness, unless students make a conscious effort to fight it. Which I'm sure many of you do, but you must have seen clasmates that, by third year, are already strutting around like tin-plated martinets and treating their patients the same way they've been treated by their instructors.
Do some schools have a system like this? I believe they do. But not the one I went to...