It isn't the sequences or even the outward shape of Tai Chi so much as what is happening within the body that generates the shapes and sequences. There are long-term, transformative effect on the body. For example, my own musculature have developed structures you cannot develop through weight-lifting.

I can tell you for example, xingyiquan, baguazhang, bajiquan, wing chun, akijutsu (among many others) all (historically) share a common set of practice principles (yijinjing) with taijiquan, yet what ends up happening under the cover are vastly different. For example, at some level of practice, taijiquan is like working with a beach ball floating on water, but baguazhang is more like winding up wires in different planes of motion. Yet both share a smaller set of common principles.

Although the forms and sequences are now taught as foundations and the source of these changes, they are probably better taught as a capstone in which a practitioner builds up and constructs from primitives over the years of practice. There are some taijiquan practitioners who get into the art a lot deeper than the mainstream, and end up practicing holding specific taijiquan postures statically. (In fact, the historical taijiquan manuals shows the set of core postures, not necessarily the sequence of them).