If you understand the principles and concepts that provide the base of a system, you do not need to learn thousands of techniques.
You are right, you absolutely do NOT need to. But... in the original TKD I learned, there was a specific way in which they crossed their arms to block, or move, while Chung Do Kwan there was another. In mixing the two, I have found to have created an even more efficient method to do what both did, which is to generate power. The point isn't to be a walking compendium, the point is to find the more efficient method.
Substituting more techniques in leu of understanding how a system operates does not increase a person's skill in martial arts. In fact, I would say it does exactly the opposite. It retards true understanding and slows real growth as a martial artist.
I agree that it would naturally have to retard the insight... for a time at least. For example. when I switched from Moo Duk Kwan to Chung Do Kwan, despite being a 3rd dan in moo duk kwan, the time when one should be focusing less on growth, and more on refining to perfection, Is topped practicing that art completely until my ability in Chung Do Kwan had matched it. Only then did I see that while my motive to stop practicing had been wrong (Its a bad style! F it. Imma learn this one which is BETTER) the result was right; I had gained comparative insight I had in one style, within another, and stepping back, could see how both could work together. In that I learned how to harmonize a style from WTF, with its opposite, from ITF.
I wouldn't say learning all the techniques which exist, and their application, makes one a better martial artist. Knowing which of those techniques to pick, in regards to one's own subjective ability, is growth though, when it's actually conducive, rather than detracting, and destructive. It's why I advise cross training, but only when one has a clearly defined base, so they can alter and change where necessary.
It is true also, that such mixing of arts creates... something which is not the original art. I think labeling it 'frankenstein' or anything else negative conveys a lack of ability to value that not only is that how most martial will be in our much more connected, and accessible world.
But I believe at some point, the art instills so much in a person's essence, that the true martial artist eventually internalizes that art. Every movement in life reflects their art, and hopefully their style definitively through that. I constantly circle walk at work (half-circle, pivots) because it makes it much easier to move about in the constrained space. And sooner or later, the art which has become a part of the martial artist, if they grow enough as an artist, will give themself back into the art.
I see it when I look at two extremely skilled students from one group I practice with; while both have only trained in chung do kwan, and it is clear they are both employing chung do kwan, it is also clear that how they move, subjectively, is much different from the other. Around 3rd dan, once the technical aspects have been mastered and spiritual begin to play as much a role, a person begins to do their art, but it is theirs then, no longer chung do kwan. It is their name-style of chung do kwan, or whatever art or person you want to sub.
And sooner or later, when you frankenstein things, and grow, give it direction, have an internal philosophy to guide the template... something emerges which is new. It's why so many can see pieces of this style or that, of their own... but not.
And if it works, and it is something where you can place it next to where it came from, and say without doubt that it is something different, why should that 'frankenstein' art not be respected as one? We all frankenstein our arts, even if we learn every technique in it; we still only use which is what works for us as martial artists.
I've just chosen not to constrain myself to one system, in that growth. And because of that you'll find I can box using virtually every (within reason- there are always styles, and techniques which I will be unaware of, and unlearned in) move available to the hands. While the Wing chun Boxer may stick to their center line, I'll shift from the horizontal punches of shotokan, to the straight line, vertical of Wing chun, to open-palm of ba-gua, to the brawling of boxing. I can use overhand, and under, because I've had the teachers who taught me all that diversity, and it was up to me to put it together so it works. And it does.
There is a fine line in MA where confidence appears as arrogance. Technique collecting does not make one better- just more knowledge, and it is true, knowledge can clutter and inhibit. But that's why we seperate our mind from our action in martial arts, why we reach for the zen of no thought, so that we may react to any instant, in the appropriate fashion. But I refuse to stop growing as a martial artist- and if anyone thinks I am the same kind of person who reaches for the new, while neglecting to hone what I already have, does not understand who I am as a martial artist.
When people ask me how I do things, it takes me time to stop, rethink what I did. I tend to forget- my body, and my mind react instinctively, and those instincts have been trained to use exactly what is necessary when, from the plethora of what I do KNOW.
It's like... just because I know how to execute a 720, or so and so technique, does not mean I've decided to internalize it so its the natural thing I'll reach for. Chances are, despite knowing it, I'll never even think of possibly thinking about using it in a fight.
I uh, guess the martial artist just needs to train their techniques to the point it's less of a choice response, and more an instinctual response, which has already taken into account all the choices, and chosen it for me. Immediately. And it sounds hard, but I use the fly as an example; they can see at all angles, at all times, and their minds are hardwired to calculate exactly what is approaching it; it's size, momentum, and such, and instantly that simple little mind instinctively chooses out of all the infinite possible paths and choices the one which WILL be the route of escape.
And humans are one of the few creatures who can just reprogram their mind, even if over time, to become like that. I just think most people don't realize the mind is so power, and that it determines reality to the extend, in martial arts, that our lack of seeing what to do, that jamming on all the choices, is having not learned to master the mind to automatically choose the efficient, and right choice.
Sure, we're human, and err. But that's what our training is for; so we don't, when it matters.