Health

Ultimately, tai chi works on health first of all.
If your body is not strong, fit and balanced, what use is it in self defence?

You may take longer to gain combat skills, but how many other martial arts can be trained almost all of your life?
How many medical studies/journals cite the positive health benefits of tai chi?

Do you read any equivalent journals regarding judo?
Maybe, but it seems doubtful.
In some martial arts you are deemed 'a veteran' by the age of 40. You gracefully retire and become a judge/referee.