Wish to go against my supervisors in the half-yearly review report. Is it suicidal for my PhD?
Getting your PhD in good time is probably the most important for your career, so you should focus on that.  It would almost certainly delay you to change topics now, and adding extra subjects "on the side" may also endanger your timely completion of the core work.  And plus, as you are aware, you are highly dependent on the goodwill of your supervisors in this situation.

Assuming a change of topics would introduce a one-year delay in your studies, there are alternative ways you could use that year to build your CV:


Work in a postdoctoral research position on a more "practical" topic.
Work in industry for a year in a lower-level position.


Both of these would likely be more effective for your long-term goal than switching topics now.  And, in both cases you would be paid and wouldn't have to worry about your funding running out.  Of course, you would have to find such a job, but I still think it is lower risk than trying to change topics now against the will of your supervisors.

Also, your fears about not being able to get a job because the topic is too theoretical may be unwarranted. Generally, most of the specific things you do in a job are learned on the job.  Employers will look at your education for: 


Evidence of your ability to learn and succeed.  Simply completing your PhD (and doing it well) demonstrates this.
A good theoretical background in the field.  Working even on an impractical "Artificial Intelligence/Machine Learning problem" should show this.


In short, focus on doing an excellent job with your current PhD topic; there are still logical steps from there to your goal.