Martial athlete

Many people who commence taijiquan practice are essentially 'daydreamers'.
They have fanciful notions of becoming a martial artist but entirely lack the grit and determination required to accomplish the task.
Instead of committing to a challenging regime of on-going comprehensive, rigorous training, the student is contented with the dream.
Combat is not easy and there is a risk of injury if the student is unfit. This is true of any martial art.

To reach a high level of skill, the student needs to take a lesson from sport. 
They must become a martial athlete.

Is tai chi compatible with the gym?

Gym work typically shortens muscles and promotes patterns of tension and fixity in the body.

Tai chi does the opposite: it allows muscles to lengthen, releases tensions and encourages good skeletal use.
The muscle work that takes place in tai chi is principally geared towards whole-body movement and training the body to rely more upon postural muscles.
 

Body building and weight lifting are fashionable activities today. The emphasis is upon developing external muscles which creates an armouring effect that can eventually distort the bony structure. It is the over developed musculature that actually torque's the bones and discourages them from bearing additional weight. The body attempts to compensate and problems arise.

Running does not necessarily in and of itself improve posture that is already poor and constricted. It often exaggerates problems due to the substitution of inappropriate muscles. The repetitive inappropriate development of the musculature (as in body building or weight lifting) often leads to diminished sensitivity. Stress occurs in the knees and lower back, encouraging injury.

Various sport activities emphasize strength, endurance and speed. Development of muscle control rather than skeletal balance takes precedence. Gaining speed at the expense of mounting tension, is too often the goal. 

(Liz Koch)

The health benefits of tai chi are quickly undermined by weight lifting, exertion and forcing.

How many people study a martial art or a fighting art?

Out of those people that exercise, how many choose to study a martial art or a fighting art? 
Not many.
Sport is far more popular than martial arts training.
Running, football, the gym, yoga... these more widely practiced.

Not many people study the martial arts these days.