ADD: I have never ranted about classical techniques or a solid classical training being outdated - ever, anywhere anytime. I should really demand satisfaction or that you retract that one immediately
But I never said it and will not say it. [Show that statement to me, please.]
The 'online theory community' you imagine exists is like this and all other open admission forums - completely uneven as to what and who you get, meaning you still 'have to know a bit' before you select the more creditable information or advice. It is truly a free-for all, which is in one way great, but in this instance, not The Source in any way shape or form. END ADD:
The internet is not a teacher, it just has gobs of information: many 'free' tutorials are literally useless and many tutorials or online teaching for a fee are being taught by those with no real expertise (to hell with credentials, just talking about someone who knows the material and knows how to communicate it.)
Then are we talking pop theory, which is often confused with classical theory, and the huge difference between the two? There is no 'C7' in classical theory, it has to be a roman numeral, designating scale degree, and Arabic numerals denoting which position the chord is in. We should throw jazz theory in the mix too. The three sorts of theory suit the needs of each genre, but there is pretty much a no twain shall meet factor there -- often their terminology is at fundamental odds with the terminology of the others.
Younger people tend more to believe everything on 'teh internetz' is good and 'legitimate.' Wikipedia is phenomenal, but is also phenomenally unreliable and often enough downright wrong. (What? Wikipedia incorrect?)
Lets say the internet is more like a calculator - you have to have an idea of what is correct to begin with, or you can push the wrong button and think and take / mistake an incorrect answer (translate information) as good.
A lot of the phenomenon of the exponentially spreading idea one can 'self-teach' is in itself a result of internet myth, whether it is instrumental performance or study of theory. Theory, unlike math, has dozens of 'correct' answers for any problem presented. The most concrete of premises or procedures has dozens of ways of being approached and the problems present solved: Theory, even beginning levels, is that much in flux. If you do know of sites where you get that kind of tutelage, low cost or free, please list them for anyone in need, such as the gentleman asking here.
There is absolutely NO SUBSTITUTE for a one on one interactive situation, where a question can be immediately addressed, the student's thinking is already taken into account as part of the tutor's approach. Some qualified teachers may be available online, for a fee, where it is set up like an old-fashioned correspondence course. Those, if existing, are the optimum the internet could offer. The good ones are not 'free' and often cost nearly the same as taking a course or hiring an in-person tutor.
To think otherwise is to really think all people are equal and all cultures are of equal merit - and anyone can become anything, evidently without the preparation the real ones all have had. Good luck with that one.