Did you start this here or was it just immediately moved by the mods?
everything wrote:- it is "better" in every way
- it should only be used alone as a single style or else it's not this "better" art
- if you use anything else you're not "high level"
- training other arts will confuse it.
these assumptions aren't applied to bjj, wrestling, boxing, muay thai, judo, etc.
What I'll say concerning these assumptions is that Taijiquan is unique, complete, and difficult to attain.
"Better" is a meaningless term without some qualification.
Better for health than any other martial art? Unquestionably so in my opinion, show me another martial art that someone in a wheelchair can practice.
Better for general self defense? Depends on how diligently one practices and the quality of the instruction they receive. I don't feel that it takes 10 years to get any fighting benefit, but it definitely takes good practice.
The other two points, well, go back to my first point. It's unique, it's not like other styles, even Neijia, at a deeper level. It's got some similarities here and there with other arts, but it's also got some unique aspects that no other art has.
It is a complete art. Mastery of the art includes strikes, throws, locks, and grappling.
As we were discussing in the other thread, taijiquan has answers for any question your opponent may ask, but if you haven't studied it sufficiently or progressed to a suitable point in your study, you may not have that answer in the vocabulary of your practice. Studying other arts can lead you to supplying answers from those arts instead of from taijiquan.
But studying other arts can help you become a more complete fighter, sure. Depending on where you are in your study it can have different effects.
If you're early in your taijiquan and haven't yet discovered how to use it for self defense effectively, then studying another martial art, in my opinion, will simply give you the techniques of that art. If you've gotten a sufficient level in taijiquan then studying another art is simply another expression of your taijiquan, in my opinion. Once you've got the taiji in your body, everything is taijiquan. Before you get it, you run the risk of delaying progress by spending too much time in another system that you could be spending in your taijiquan.
The Gracies were of the opinion that all you needed was BJJ.
I think there is too much emphasis on UFC style competitions these days. That's not the be-all, end-all of martial arts expression, and honestly isn't even a very common self defense situation.