The big problem here is that too many people believe, because they have been so told, that martial arts skill development only comes from a teacher (Sifu/Sensei/Guro/Guru/Kru etc.) transmitting the information to the student. The passage of real info only occurs once per generation, and only one real successor is chosen. The transmission of the style's secrets (and secrets they must be) only occurs after many years, perhaps preceding the death of the master by the shortest possible period.
This seems more prevalent to Chinese styles than any other, though this may only be my opinion coloured by decades of involvement and training in those styles. We even have situations like that claimed in one version of Yip Man Wing Chun where Yip Man's teacher, Chan Wah Shun, allegedly was only taught a modified and crippled version of the style. This after he originally tried to learn the style by watching his teacher Leung Jan teach his (the teacher's) son from a secret hidey hole. Leung Jan of course could always tell when Chan was or was not watching and taught his son the modified version when Chan watched, and the real version when he wasn't. Eventually Leung Jan agreed to teach Chan, but continued teaching him the fake version, not the real thing. Must have been very confusing and hard to manage ... lies usually are, just watch Fawlty Towers.
The problems with this are legion. With only one successor, the odds of styles getting lost are not insignificant. If you are the master, and the guy you chose as successor turns out to have issues (and a case can be made that many of the alleged successors have such issues), I guess you are honour bound to take the sacred knowledge to the grave with you. And what if your guy has an accident or dies in one of the challenge match of which there were supposedly plenty in the pre-internet era but rather fewer now?
Not a great business model, is it? Only one student? He's going to pay your bills? Or take on a whole lot of patsies, bilk 'em by showing 'em crap and only select one near the end to show the real stuff? Better hope like hell he's just stupid enough to put up with this, but not much stupider. Or he doesn't get greedy. Or smart.
Aside: I actually went to a Kwoon where a few of the students tried to sue the instructor because he wasn't showing them the "secret" stuff and thus not providing the goods that were advertised for sale. Surprise! It didn't work.
Secrecy itself presents a problem. Like everything else, fighting techniques require exposure to resisting opponents to determine whether or not they really work, what effective counters might work, and how to change the technique to lessen the effectiveness of such counters, or to counter the counters. The parallel here that comes to mind is cryptographic algorithms; the best ones are fully publicized, and the best cryptographic minds around the world invited, indeed encouraged, to hammer the things, to try to find weaknesses or shortcuts. You can have a pretty good level of confidence in an algorithm that is still standing after years or decades of the best cryptanalysts on the planet trying to knock it down.
If the fighting style has secret techniques, particularly those determined to be too deadly to spar with, then their effectiveness is never really tested.
If you've only got one guy showing the full system to one other guy, because of some cosmic rule set down by no one that says you can't have several successors, then it eventually becomes like making a photocopy of a photocopy of a photocopy of a ... bits are forgotten or misunderstood, the quality slowly but surely and inevitably leaches away, a very long and complicated game of, no pun intended, Chinese Whispers.
Nearly all other fields of human endeavour work differently. Knowledge rarely springs fully formed from the void, appearing as the result of a dream or hallucinogenic vision. There are exceptions - Kekule, the dream of thee snake biting its own tail and the benzene ring, but these are rare. And Mr Kekule ws well and truly putting in th 99% perspiration that allowed him that 1% of inspiration. Knowledge is build on previous knowledge, tested and peer reviewed. The lone genius exists but is the rare exception. Generally knowledge increases because previous knowledge is built on or refined. "If I have seen further it is only because I have stood on the Shoulders of Giants" - Isaac Newton. Mr Newton was not exactly a dumbass.
You wanna be a martial artist? Go sit at the feet of some old guy in a cave or temple somewhere and hope like hell he's going to pick you and not some other dude with more talent, a smoother presentation or deeper pockets. Good luck.
No. Go somewhere where an instructor will show you the basics. While you want to be sure he knows what he's talking about if he's taking your money, your success in martial arts will be pretty much up to you, not the instructor.
In Kung fu, there is a set of stone tablets that supply the totality of the system. No one other than the founder has ever been smart enough to tinker with, let alone try to improve or upgrade the system. having the arrogance to try, or to mix other influences, is to start down the road to perdition and failure.
In non traditional MA's, you need to have a basic understanding of the fundamentals. But after that, if you aren't using your brain and attempting to solve your own problems using your own unique experience and initiative, you aren't going to get very far. While unfiltered input at the start is confusing, once you get the fundamentals understood you damn well better start using that brain in your head. Martial arts is about self discovery, not reading someone else's lecture notes.
The TMA guy will excommunicate you, cast you into outer darkness, for daring to seek another's opinion. Go train with them, you'll have embraced the dark side. Loyalty and truth are important, and it's only polite to tell the instructors in your life what you are up to and who you are training with, but generally a wide range of experience is a good thing. Certainly more interesting.
You are the master of your fate. You don't want to abrogate that responsibility to anyone. It's called SELF actualisation for a reason, because you have to do it yourself. There are people who can and will help you, people who have walked a path similar to yours. But you don't want to become them or their idea of a disciple or descendant. Take the helm, navigate your own course.