@ smeaper-
Music teachers come in all stripes. There are music teachers who are discouraging or make the kid feel like they are no good because they subscribe to the notion that giving compliments ruins a student (stupid, prevalent in some cultures more than others, but still stupid), old time violin teachers were infamous for this.
What you saw there, though, is someone who is bitter because their own career never took off and thus is telling everyone “It is impossible to make it, forget it”. I also will note that someone subbing in music theory at a school is not exactly someone who is in a position to say something like that, as my father used to tell me when I got upset at someone criticizing me, ‘consider the source’.
What is funny is the same people who will tell you that you should not go into music, that it is so hard to go into, will say things like "you should go into computers’ or “you should go to law school” or “you should become an accountant”. The funny part is these days those are not necessarily instant road to a good job, with technical jobs (and with accounting), the routine jobs, especially entry level, have often been outsourced overseas or given to foreign visa holders, and it has meant that to get a foothold or expect to make good salaries in those positions is not what it once was, salaries and opportunities both have been suppressed, and to get those jobs and do well you need to be among the best, not ‘in the crowd’, and music is very much like that, it takes someone with the skills and talent and desire to make it, however you define that…not to mention that the idea college is about job training skills fails in many regards…
Studying music is a bachelor’s degree that brings with it a lot of skills and abilities that other areas of college study don’t bring. When you study music, you deal with ambiguity, take a test in calculus and asked to solve a problem in differential calc, you know what to do, whereas with music when you walk into an audition or jury, you don’t know how/well you will be evaluated, yet you have to still try and prepare. Take an academic class, and it is relatively easy to know how much to study for it, you know the breadth of the material, expected answers, kinds of questions, with music how much practicing is enough? What are you measuring? It isn’t very clear, and takes a lot of self study, and taking a stab at the ambiguous and otherwise not well defined…and for academic rigor, things like music theory, ear training, music history and the like are not easy. Voice students also have foreign language study, so when people say ‘it doesn’t teach anything’, they are full of it, pure and simple.
Like I said, someone doing sub work in music theory is not someone who is in a position to judge.